2.0 Flashcards

Question

What is an element?

Answer 1

Kind of matter that cannot be decomposed into two or more simpler types of matter

Answer 2

``` K 2 (greatest electron binding energy) L 8 M 18 N 32 O 32 P 32 ```

Answer 3

Electron receives sufficient energy to raise it to a higher energy level. Less than 10eV energy needed.

Answer 4

Electron receives sufficient energy to overcome the binding energy to escape the atom. In the order of 10eV

Answer 5

V (freq) = c / λ c = speed of light 3 x 10^8m/s λ = wavelength Speed remains constant so wavelength and freq must vary together. High freq + short wavelength

Answer 6

``` Gamma rays 0.0001nm XRs 0.01nm-10nm UV Visible light 400-740nm Infrared 740nm - 0.7cm Radiowaves 1mm- 100km ```

Answer 7

E= hc/λ ``` h= plancks constant 6.63 x 10^-34 c= speed of light 3x10 ^8m/s λ= frequency ```

Answer 8

Becquerels (MBq/GBq)- SI units | Curies

Answer 9

2 protons + 2 neutrons Charge +2 Velocity 6% speed of light

Answer 10

Either an electron or positron

Answer 11

XRs are created by accelerating electrons hitting a target and by electrons moving between atomic shells (characteristic X rays) Gamma rays are emitted from nuclei of unstable isotopes (unable to change the rate of production or energy) XRs can have higher energy than gamma and vice versa

Answer 12

Range of photon energies are produced: - continuous spectrum- results from Bremstrahlung and depends on energy of incoming electrons and on atomic number - Discrete spikes of particular energy - particular to target material and are characteristic part of the XR spectra Provides info on the quality of the beam (ability of beam to penetrate an object)

Answer 13

Electron source- eg tungsten wire heated by high electrical current -> thermionic emission (electron cloud) Target -> high atomic number and melting point -> Bremstrahlung Power supply -> generated positive charge on the target Higher the voltage, higher energy XRs produced Higher charge through cathode = higher intensity Higher Z number of target = higher intensity

Answer 14

Loss of photons as beam penetrates some materials. (Compare that to charged particles: photons lose number of photons so are ATTENUATED, charged particles reduce in energy so think about energy transfer/stopping power) | = absorption + scatter

Answer 15

I = Io x e^ -µx ``` Io = intensity beam on entering material µ= linear attenuation coefficient (m-1) x= thickness of the material ```

Answer 16

Linear attenuation coefficient (µ) = fraction of attenuated incident photons in a monoenergetic beam per unit thickness of material (m^-1) Mass attenuation coefficient (µ/p) - divide the linear attenuation coefficient by the density of the absorber (p), units m^2kg^-1 (per unit mass rather than unit path length) Both vary with energy of the beam

Answer 17

The energy at which the photon energy equals binding energy of K (inner) electron shell. the K-edge is a sudden increase in x-ray absorption occurring when the energy of the X-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons. Results in sudden attenuation. Due to photoelectric effect

Answer 18

Effective atomic number is 9 <25kv photoelectric 25kv - 25mv compton scatter >20MV pair production

Answer 19

charge of one particle (q1) x charge of other particle (q2) / distance between their centres squared (r^2)

Answer 20

Soft collision, hard collision and radiative losses

Answer 21

Charge particle interaction Occurs when impact parameter (b) >>a (atomic radius) - particle is not passing near the atom Small amount of energy transferred to orbital electrons: - excitation of atomic electron to a higher level which returns to ground state with emission of a photon - ionisation of atom by excitation of a valence electron-> transfer a few eV energy to a medium Accounts for half of energy loss but by far most common interaction just small amounts of energy lost

Answer 22

In a certain material highly energetic charged particles can travel faster than the speed of light- very small part of energy (<0.1%) of soft collisions is emitted as a coherent bluish white.

Answer 23

Charged particle interaction Occur when impact parameter similar to atomic radius (b=a) High speed electron knocks out an inner orbiting electron Vacancy filled by either: - electron dropping down emitting photon - energy transferred to an outer electron emitting it from the atom -> auger electron Possibly delta ray ejection- outgoing electron has sufficient energy to produce secondary ionisations

Answer 24

B<95%- elastic nuclear scatter interaction- electron deflected without losing energy 2-3% cases - charged particle passing near a nucleus is deflected with sudden deceleration due to couloumb force and loss of energy -> Bremstrahlung (braking radiation) -> energy lost emitted as photon Main source of medical XR radiation In low atomic number materials little energy lost in this way

Answer 25

``` Stopping power (property of material) - loss of energy/unit distance by charged particle Linear energy transfer (property of radiation) - gain of energy/unit distance of media from the charged particle ```

Answer 26

The rate at which energy is lost along a charged particle track. Units = J m^-1 Usually shown by dE/dx

Answer 27

Graph dE/dx vs distance of penetration Low constant rate of energy loss immediately after entering a medium. Towards the end of the path, the rate of energy loss rises dramatically (Bragg peak) then falls to zero

Answer 28

Proportional to square of a charged particle (i.e. an alpha particle (+2) loses energy 4x as fast as a proton Inversely proportional to the square of the velocity (as particle slows down its energy loss increases) Independent of mass of the charged particle (rate of energy loss of proton and electron at same velocity will be similar) - not sure that is true - think higher mass = higher linear energy transfer.

Answer 29

As electrons will scatter and many end up travelling in the direction which they came from. Protons are heavier so not as easily deflected.

Answer 30

Mass collision stopping power (through hard and soft collisions) + mass radiative stopping power (through bremstrahlung)

Answer 31

Electron density of material (lower -> less collisions) | Energy of incoming particle (more energy lost by low energy particles)

Answer 32

Proportional to: Square of atomic number of material Energy of incoming particle Inversely proportional to square of mass hence only occurs with electrons and positrons

Answer 33

The rate at which energy is imparted to the medium along a charged particle track (keV/um) Also known as restricted linear stopping power as equal to collision stopping power after excluding secondary electrons with energies larger than a value. Draws attention to energy lost by electrons that is absorbed in close vicinity to electron path rather than the total energy dissipated by the electron (removes some delta rays)

Answer 34

Particle type - alpha particle LET=50, 10kev electron LET=2.5, 1MeV electron LET=0.2 Particle energy - high energy particles have low rate of energy loss and small amount of ionisations, as particle loses its energy LET increases. LET increases with mass andcharge

Answer 35

Elastic scattering- neutron interacts with nucleus as a whole. Nucleus gains kinetic energy and recoils through medium. Original neutron loses energy and is deflected from its path. Inelastic scattering- occurs when a neutron is absorbed into a nucleus -> nucleus will be unstable and several different phenomona can occur: - eject neutron - eject proton/alpha particle/large nuclear fragment -> high LET - eject a high energy photon

Answer 36

>8Mev (binding energy of nucleon)

Answer 37

Distance to bragg peak

Answer 38

Use various different energy beams to form a spread out bragg peak (SOBP)

Answer 39

Low LET radiation with a tiny high LET portion at terminal track

Answer 40

Cyclotron or synchotron | Hydrogen gas + heat -> plasma + electrostactic force -> protons

Answer 41

Mean energy deposited (dE) in a medium of mass (dM) by ionising radiation dE/dM What you prescribe 1 Gy = 1 joule/kg Relevant to all ionising radiation- directly and indirectly

Answer 42

Number of ionisation events measured as an indication of deposited energy in air. E = Q/m in Coloumbs per kg (C/kg) Q is total charge of ions produced in air when all electrons liberated by photons in air of mass (m) are completed stopped. ONLY APPLIES TO PHOTONS Exposure is the amount of charge liberated per kilogram of air by an X ray beam – i.e. the number of ionisations of air particles

Answer 43

``` ONLY PHOTONS ONLY IN AIR NEED ELECTRONIC EQUILIBRIUM couloumbs per kg Used in ion chambers ```

Answer 44

Measure exposure

Answer 45

Describes the photon transferring its kinetic energy to an electron -> energy transferred from indirectly ionising radiation to directly ionising radiation. how many electrons are activated by a photon beam in a given mass. Kerma = Etr/m Etr is sum of initial kinetic energies of all charged particles liberated by uncharged particles in mass APPLIES TO PHOTONS IN ANY MEDIUM (not just air like exposure) kinetic energy released per unit mass

Answer 46

Energy transfer from photon -> electron At a single point Interactions occurs IN MEDIUM (even if ionisation particle leaves the volume) J/kg

Answer 47

Collisional kerma- kinetic energy expended in inelastic collisions (ionisation and excitation) with atomic electrons Radiative - kinetic energy expended in radiative collisions with atomic nuclei (Bremstrahlung) For low-energy photons, kerma is numerically approximately the same as absorbed dose. For higher-energy photons, kerma is larger than absorbed dose because some highly energetic secondary electrons and X-rays escape the region of interest before depositing their energy. The escaping energy is counted in kerma, but not in absorbed dose.

Answer 48

Air kerma is an expression of exposure in terms of energy rather than charge Exposure can be measured directly whereas kerma canot Air kerma (j/kg)= exposure (C/kg) x Wair/e (J/C) Wair/e =average energy required to produce an ion pair in dry air (33.97 J/C)

Answer 49

Number of particles (N) incident on a sphere of cross sectional area (a)> = N/a SI units M^-2 Indepedent of radiation direction

Answer 50

Energy carried by these particles Radiation energy (R) entering a sphere of cross sectional area (a) R/a SI units J m ^-2

Answer 51

At surface as most photons at surface. | Photons attenuated by the medium to kerma decreases with depth

Answer 52

Dmax (built up region prior to this) | Electrons continue to travel a short distance before depositing their energy

Answer 53

ONLY energy absorbed inside the medium

Answer 54

Energy leaving the material = energy entering the material Can assume absorbed dose = colllisional kerma For energy <300Kv can assume radiative losses negligible however above this we cannot. Closest we get to CPE is at dmax. Beyond this kerm and dose curves divide depending on beam energy and transient charged particle equilibrium exists (TCPE)

Answer 55

photon energy atomic number of material electron density of material

Answer 56

Change in kerma is proportional to mass energy transfer coefficients As kerma changes so does the absorbed dose, according to the ratio of the mass energy absorption coefficient of two materials Kerma will change in a discreet step Absorbed dose will change more gradually due to backscatter (example of electronic disequilibrium)

Answer 57

Energy absorbed per unit mass in two different materials subject to same photon fluence will be proportional to their mass energy absorption coefficient- ratio: (μen/ρ)med/(μen/ρ)air ρ = density μ = linear energy absorption coefficient For materials with low atomic number the mass energy absorption coefficient does not vary much with energy. For materials with higher Z it is higher a lower energies where photoelectric effect is more probably and as increases it drops as compton scatter becomes dominant interaction and electron denisty become more important

Answer 58

Heat (calorimetry) Light (scintillator) Ions -> electron current (ion chambers) Electron/positron pairs (diodes) -> electric current

Answer 59

``` Change in temp determines absorbed dose Absolute dosimetry Energy J = mc (T2-T1) Mass (m) in kg c = Specific heat capactiy of medium (J/kg per centigrade) T1, T2- initial and final temp Therefore dose (gray) = c (T2-T1) Absolute detector Not used in hospitals as very large - but is used by the NPL as gold standard detector Graphite used as low heat capacity so heats up a lot for a small amount of radiation. ```

Answer 60

Free air ionisation chamber thimble ionisation chamber -> Farmer = photons Paralell plate chamber = electrons well-type ionization chamber = brachytherapy Geiger muller counter

Answer 61

Photons enters air volume (metal box) and interact to form secondary electrons. Electrons travel to anode, positive ions to cathode and electric current is measured. Electrode separation must be sufficient that electrons completely stop in air (cause ionisation) before reach (proportional to exposure) - E<250kev = 20cm, E>1Mev - several metres Must known mass of air from which collecting electrons Must have charged particle equilibrium to make sure exposure = kerma (enough built up to primary interaction area). Charged particle equilibrium = particles in = particles out so kerma (photons to electrons) = absorbed dose/exposure (electrons to ions) Absolute dosimeter - very large so again not used in hospital - only used by NPL.

Answer 62

Dose (Gy) = energy to produce one ion pair x charge collected / charge of one electron x mass air (kg) J/Kg

Answer 63

Small cylindrical chamber Graphite wall = cathode. Graphite has similar atomic number to air but density x 1000, produces an electron density similar to free air chamber- enough to achieve charged particle equilibrium Photon comes from outside and hits graphite wall - turns to electrons which travel through inner air to reach the anode. Anode- central wire - collects the electrons Measurement volume 0.6cm ^3 Operate at 200-300v Farmer ionisation chamber is one specific type of thimble ionisation chamber. Small so used in hospital but needs calibration - not an absolute dosimeter.

Answer 64

Calibration of linac | Quality control and commissioning measurements

Answer 65

Cannot be exactly sure of measurement bolumber so need to be calibrated against an absolute dosimeter (send to national institute for physics) who provide a correction factor

Answer 66

reading x calibration factor x temp/pressure correction factor x ion recombination factor temperature and pressure correction factor = room temp in kelvin (+273) / 293 x 1013 / room pressure in mbar

Answer 67

+ ves : size, linear response to dose, energy response equivalent of air -ves: not absolute, requires calibration, ion recombination must be corrected for, high spatial resolution measurement difficult due to chamber size (measurement volume too large for field size <4cm, inaccurate at steep dose gradients)

Answer 68

2 parallel plates (electrodes)- 2mm apart Wide guard ring ensures no in-scattering Allows measuring point to be much better defined in space and to have finer measurment resolution - this is important for electrons as their depth-dose curve alters a lot over 5mm (width of a farmer chamber) but wont change much over 2mm (parallel plate chamber) Requires correction factors like thimble

Answer 69

Electrons | Surface/build up measurements with photons

Answer 70

Gas chamber with a high polarising voltage across electrodes (900v). Central anode- high voltage Ionisation of electrons -> ionise another gas molecule-> avalanche effect -> entire tube ionised for each single photon event Measures dose rate not dose Very sensitive Used for radiation protection purposes - e.g. if you drop a brachytherapy seed, use this to try to find it - audible indication of dose rate

Answer 71

Advantages- sensitive, real time measurement, indication of intensity, portable Disadvantages- saturates at high dose rates so you get a "dead tube" whilst waiting for it to reset, CANT determine dose, energy or type of radiation

Answer 72

NOT a gas chamber Scintillation crystal (often sodium iodide) used- photon hits crystal and causes an emission of light. Photocathode converted each photon to an electron -> photomultipluer tubes multiplies electrons to create a cascade which is measured as current. Measures dose rate. Radiation protection.

Answer 73

Advantages: can measure energy as well as number of photons. Disadvantages: need airtight container for crystal, sodium iodide only picks up high energy gamma (No beta or alpha) Basically used the same as Geiger counter

Answer 74

Relative dosimetry of electron and photon beams In vivo dosimetry Quality assurance measurments

Answer 75

Silicon diode detectors | TLDs

Answer 76

``` Silicon crystal (semi-conductor) has 4 electrons in its outer shell -> forms covalent bonds with neighbouring atoms -> crystal lattice If crystal absorbs energy then to bonds can break and leave a free negative electron and a positive hole. Impurities added to the crystal increase number of electrons or holes- called doping. -> impurity has 5 outer shell electrons eg phosphorus- extra electrons -> negative charge -> n type -> impurity has 3 outer shell electrons eg boron -> extra positive holes -> p type Electrical field naturally formed over the depletion zone (no need to apply high voltage) Ionising radiation produces electron hole pairs -> electrons move towards n-doped region (as the edge of it is deplete of electrons) and "holes" move towards p-doped region = current Current proprional to ion pairs proportional to dose ```

Answer 77

Advantages - very sensitive as very dense - small 0.3mm3 measurment volume- good resolution for small fields - no voltage required - hence can be used for in vivo dosimetry - used for PDDs in patients - instant read out Disadvantages - poor tissue equivalence as atomic number is very different to tissue (much higher) so get more photoelectric effect so overdetect low energy photons - over response to low dose radiation due to photoelectric effect - gradual radiation damage and loss of sensitivity - dose rate dependent - not as reproducible as ionisation chamber - relative dosimetry - sensitive to temp- keep st skin temp

Answer 78

Thermoluminescent dosimeters (TLDs) Small chips of solid state materials. When irradiated crystal absorbs energy and free electrons migrate in the lattice -> get caught in traps. Later TLD heated and trapped electrons gain enough energy to be released and recombine with the positive hole -> visible light is emitted. TLD reader converts the light into an electrical signal using a photomultiplier tube So irradiate then heat - produces light - proportional to radiation dose

Answer 79

``` Advantages - small (tiny) - no cable or voltage - detects wide dose range - reasonable tissue equivalent - in vivo dosimetry if you don't need result immediately - radiation detectors - in detectors worn by staff for 3 months and sent off to be read Disadvantages - time delay before readout as need to warm up first - readout and calibration time consuming - delicate, small and easy to lose - accuracy limited at 5% ```

Answer 80

dose measurement device consisting of many ionisation chambers or diodes. Allows measurement across a 2D grid. So can measure the beam flatness/symmetry.

Answer 81

Beam quality assurance MLC accuracy IMRT verification Disadvantage: expensive, resolution inferior to film.

Answer 82

``` Radiographic film (film goes darker) Radiochromic film (colour change) ```

Answer 83

Thin flexible plastic sheet coated in radiation sensitive emulsion (eg silver bromide). Radiation causes ionisation on film-> causes darker. Need a dark room and optical densitometer to measure Not used anymore

Answer 84

``` Advantages: - good spatial resolution - permanent record Disadvantages - need dark room - single use - delayed readout - calibraiton required ```

Answer 85

Changes in colour on exposure to radiation. Colour change caused by polymerisation of dyes embedded in an emulsion layer coated on a substrate Measured using a densitometer Takes 6hrs to develop Darker = more dose. Used in commissioning new machines Advantage: Very good spatial resolution. Disadvantage: expensive and single use. Very high resolution so used in SABR verification (Array is too low resolution)

Answer 86

Absolute (primary standard): kV- free air chamber mV- calorimeter Reference (Secondary and tertiary standard) - Farmer chamber - parallel plate- electrons - well type - brachytherapy Relative dosimetry (quaternary) - TLDs - solid state

Answer 87

Primary standard (nationally maintained) Secondary standard - local standard used to calibrate the tertiary/quartenary equipment Tertiary equipment- farmer chambers Quartenary equipment - TLDs, diodes

Answer 88

Superficial 50-160kV (10-100kv) | Orthovoltage 160-300kv (100-500kv)

Answer 89

Half-value layer (HVL) is the thickness of a material required to reduce the intensity of an x-ray or gamma ray to half its original value. It is used to describe beam quality of kV photons. Quality described by TPR 20/10 for MV photons and RD50 for electrons. Not suitable as it is slowly varying function of energy and it can be affected by pair production at high energies. Use water to specify beam energy: - Tissue phantom ratio/quality index - HVL and narrow beam attenuation coefficients in water - PDD

Answer 90

usually defined on central axis of beam (CADD) - a quotient of the absorbed dose at that point, divided by the absorbed dose at the depth of max dose. PDD = Dd/Dmax x 100%

Answer 91

Collimator scatter Phantom scatter- backscattered photons from within pahntom High energy electrons produced by photon interactions in air

Answer 92

Electronic disequilibrium Secondary electrons travel downstream from the build up region and are NOT replaced by electrons generated further upstream At dmax reach electronic equilibrium

Answer 93

Electrons deposit energy where the photon interaction occurs - as low energy so don't travel far. | Therefore kerma = absorbed dose

Answer 94

Attenuation Inverse square law Both affect photon fluence

Answer 95

Charged particle equilibrium

Answer 96

SSD Beam energy (higher energy = lower surface dose, deeper D max, and slower drop off after D max) Field size

Answer 97

Effect decreases Larger SSD = slower drop off, Small effect with larger SSD: deeper Dmax and lower surface dose as less low energy scattered photons/electrons as they don't get that far

Answer 98

Higher as less effect of inverse square law and it is percent (the actual dose would be higher if nearer)

Answer 99

(101.5/131.5) ^2 x 600 = 357 cGy/min

Answer 100

Lower surface dose, deeper D max (1/4 of energy) and slower fall off - as less attenuation (but inverse square law component stays the same) (Lower surface dose relative to Dmax as Dmax is prportionally higher with higher energies) | As energy increases the attenuation decreases- > increased photon fluence -> increased PDD at depth

Answer 101

Probability the photon will interact Properties of material: density, Z, electron density. Properties of photon: quality | Decreases with increasing energy

Answer 102

Bigger field size -> greater proportion of dose from scatter (scattered electrons and low energy photons and scattered photons from sides), higher surface dose (more scattered electrons and photons through collimator), shallower fall off (more phantom scatter coming into midline) (Small effect: dmax is deepest at 5x5cm although mostly stays the same: smaller than that causes less phantom scatter so less inward or backward scatter, larger than that increases electron surface scatter so brings dmax to the surface) Magnitude of effect more pronounces at kV energies as scatter in all directions and therefore contributes more to central axis dose. At higher energies scatter is forward.

Answer 103

Can't use for isocentric treatment as no fixed SSD. Instead have a fixed SAD (source axis distance). PDD is a function of SSD (varies depending on pt contour and gantry) Instead use TPR (tissue phantom ratio)/TMR (tissue maximum ratio: dose at depth/dose at dmax.)

Answer 104

TPR: used to calculate relative dose at a given depth in isocentric treatment. - ratio of dose delivered at depth d to dose delivered at reference depth dref at same SAD but surface of phantom has moved eg TPR 20/10 - ratio at 20cm and 10cm Tissue maximum radio = where the reference depth is dmax

Answer 105

Describes what happens when you change the field size 10x 10cm field size the output factor = 1 OF = dose at dmax for field size of interest / dose at dmax for reference field size Takes into account - collimator scatter - phantom scatter

Answer 106

Arrays: Ion chamber | Diodes

Answer 107

photons that pass through the middle of a flattening filter go through more metal and are hardened so will deposit more dose more deeply. Means at shallow depths- dip in centre of beam and as moves further this reverses.

Answer 108

Variation at edge of beam due to photons travelling through the collimator

Answer 109

Lateral distance between 80% and 20% dose at isocentric plane at depth Made up of transmission penumbra (transmission attenuated through collimators) + geometric penumbra (As source has a size) + scatter penumbra (inpatient scatter outwards)

Answer 110

Penumbra at any depth due to the geometry of the set up (due to the finite size of the X ray source of focal spot size)

Answer 111

Focal spot size and shape - source size -> bigger source size the bigger the penumbra SSD- increase in SSD increases the penumbra Shape and properties of collimator- increase in SCD decreases the penumbra (further down/away collimator the smaller the penumbra) Geometric penumbra originates from the radiation source when it is not a single point.

Answer 112

- compensate beams from non- orthogonal (right) angles - compensate for changed in surface shape (missing tissue externally e.g breast) - compensate for changes in depth dose fall off (missing tissue internally e.g. lung)

Answer 113

Angle between the wedge isodose lines and line perpendicular to the central axis of the beam. Should be defined at depth of 10 cm

Answer 114

Customised blocks (cast from low melting point alloy-lead)- fitted in the linac head which shield critical structures.

Answer 115

Advantages: - simple - good penumbra - excellent spatial resolution Disadvantages: - heavy - alloys can be toxic - manufacturing time - dose not zero under block - affected by field size, scatter from patient, block size and position, depth of interest

Answer 116

Radiation transmitted through the leaf of material <1% | Radiation leakage between adjacent leaves - inter-leave leakage <2%

Answer 117

2 banks of leaves, driven automatically but independent motors. Step or tongue and groove design to reduce interleaf leakage

Answer 118

Advantages: Quick no manual handling Mulitple field delivery Disadvantages: - leakage - worse penumbra - MLC direction sometimes limited by wedge direction - conformity poor

Answer 119

Block which can compensate for dose gradients perpendicular to the central axis (wedges can only provide compensation in one direction). They basically are inverses of patients to provide a uniform dose at the target. - 2 dimensional, made from aluminium or brass blocks - mounted at a distance from the patient as otherwise skin sparing lost - causes beam hardening - not used commonly now due to IMRT

Answer 120

Relative electron density of lung =0.2-0.3, water =1, bone= 1.2 1cm water = 4cm lung = 0.8cm bone

Answer 121

As enters lung there is a slight reduction in dose due to reduced backscatter from the lung. Total attenuation is less in lung due to reduced physical density which increases photon fluence distal to lung and increases dose beyond lung. As re-enters tissue the dose builds up again Bigger effect with smaller tissue sizes To calculate dose distal and remote to the lung we can use a standard correction factor Lung +3%/cm Bone -2%/cm

Answer 122

Combination of depth doses and profiles to give a 2D representation of dose

Answer 123

Increase spacing between isodoses on central axis as depth increases- > reduction in gradient of depth dose curve Widening of penumbra at depth Increased rounding of the isodoses at depth

Answer 124

Size, position and depth of target Beam data- field size, shape Absolute dose calibration of machine (dose delivered in a reference set of conditions)

Answer 125

100cm SSD Reference depth = dmax 10x 10cm field 1cGy= 1MU

Answer 126

100 x Dose (cGy) / PDD x OF (cGy/MU) x WTF x PbTF

Answer 127

Dose (cGy) / TMR x OF (cGy/MU) x WTF x PbTF x (100/100 + dmax)^2

Answer 128

2ab/ a + b

Answer 129

0.9 x 2ab/ a + b

Answer 130

Small skin sparing effect but high dose near surface and rapid fall off with small tail due to Bremstrahlung (5% dose)

Answer 131

Energy x 2 = dmax in mm

Answer 132

Energy x 3 = R90 (theurapeutic range) Energy x 4 = R50 Energy x 5 = Rp

Answer 133

Energy - increased energy = higher surface dose, deeper dmax, deeper practical range (as with more energy there is less scatter so surface dose is more similar to dmax so plateaus and goes deeper so slower fall off, whereas with small energy there is a lot of scatter causing the build up region) Field size (only effects if smaller than practical range) - generally surface dose decreases and dmax gets deeper and fall off is less step with increasing field size up to the practical range due to lateral electronic disequilibrium (think of it as you are only measuring the forward electrons so losing all the lateral electrons (so weaker beam) but get plateau for surface dose and dmax). Practical range stays the same. equivalent square should NOT be used for electrons. Electron output factors should be measured not estimated. SSD- complicated as not physical source so inverse square law cannot apply. Define a virtual source. Increased SSD = lost weak/scattered electrons so beam effectively more energy = lower surface dose, deeper dmax, deeper practical range Surface obliquity >30 degrees = brings PDD closer to skin so can get hotspots if heterogeneity in angle - need bolus to smooth out. Homogeneities: have bigger effects on electrons than photons.

Answer 134

``` Beam flaring (scatter of electrons away from central axis) esp at lower isodoses Wide penumbra At higher energy the higher isodoses converge due to increased forward scatter ```

Answer 135

Braking radiation Proportion to Z^2 of material Inversely proportional to mass of the particle (so protons don't do it) Proportional to charge of the particle. Therefore electrons and positrons the most efficient at doing it. | Energy produced from deceleration of a charged particle that passes near to the nucleus

Answer 136

0.693 / u u = linear attenuation coeffcieint Inversely proportional to it. The amount of material required to reduce the photon beam intensity to half its entrance value Used for beam quality in kV beams. Homogeneity index = first HVL/second HVL - 1 for homogeneous beams, less than 1 for heterogeneous beams.

Answer 137

multiple by the water to air stopping power ratio

Answer 138

Energy, depth and field size. Affected by attenuation only, not inverse square law | Independent of SAD as fixed

Answer 139

Cause stand off (gap to applicator)/stand in (within the applicator) Increased obliquity = increased surface dose, shallower dmax, faster falloff. This is because the electrons are produced in the same angle so don't travel as far inside the patient as they are travelling at an oblique angle for electrons. | > 40 degrees it has more of an effect

Answer 140

ACOF = ratio of measurement with an ion chamber for a 10x 10 cm field at dmax, 100cm SSD to a measurement using individual cutout at dmax ACOF = output (10x10cm, dmax, 100cm SSD) / output (cutout, dmax 100cm SSD)

Answer 141

MU = (dose/#) / ACOF

Answer 142

ACOF = backscatter factor cutout/ backscatter factor applicator x applicator factor

Answer 143

MU = (dose (cGy)/#) x (1/ACOF) x (dist/SSD)^2

Answer 144

Dose kernels to represent the dose deposited from a beam of photons spreading out due to electron scatter. Can be modelled by convolution (aka pencil beam) (1D) vs superposition (3D) - superposition is more accurate as takes into account lateral electronic disequiblibrium in different media so more accurate for heterogeneities such as lung to tissue transfer. Superposition also called collapsed cone or AAA (brand names). Deterministic = pencil beam, collapsed cone, AAA Pencil beam - started off along lines of montecarlo but then uses much quicker methods - uses small kernels created in advance during commissioning of TPS - kernels are a map of the dose that will occur for your energy of radiation beam eg 6MV hitting small volume of water - scatter form any patient can be recreated by adding up these pre-calculated small scattering volumes - process called convolution Non deterministic (based on probabilities) = monte carlo method. Probabilistic modelling of billions of electron interactions - most accurate.

Answer 145

Gross demonstrable extent of disease on imaging or examination.

Answer 146

Includes subclinical microscopic disease that has to be treated to adequate dose to achieve cure/palliation. Based on clinical knowledge/trials and specific to each tumour.

Answer 147

Includes geometrical margin for set up error and organ motion (internal margine and set up margin). Akin to a hoop to catch the tumour with.

Answer 148

Accounts of for changes in size and position of CTV | IM + CTV = ITV

Answer 149

Accounts for uncertainties in relationship between person and beam

Answer 150

Volume of the 95% isodose | Would ideally be equal to PTV but usually higher

Answer 151

Volume of tissue that receives a dose that is considered to be significant in relation to normal tissue tolerance

Answer 152

Planning organ at risk volume OAR move and are susceptible to the same random and systematic errors as CTV so we grow the OAR volume by the set up amount to create a planning organ at risk volume.

Answer 153

A point chosen within the PTV defined as: - dose at point should be clinically relevant - point should be easy to define in a clear and unambiguous way - the point should be selected so that the dose can be accurately determined - the point should be in a region where there is no steep dose gradient Ideally the point should be isocentre but not always possible

Answer 154

Level 1 reporting (not good enough for IMRT): ICRU reference point dose Max dose to PTV Min dose to PTV Dose to OARs Tumour control depends on dose to CTV, however can only be estimated by dose to PTV

Answer 155

``` Used with computerised planning GTV CTV OR PTV PRV Prescription to PTV mean or median (D50%) rather than ICRU ref point for IMRT as there can be significant heterogeneity Report PTV min and max dose using D2% and D98% Advanced treatments like IMRT - DVH ```

Answer 156

Pitch = table travel per rotation/ XR beam width eg travel = 10mm/rot BEam = 10mm Pitch = 1

Answer 157

Each pixel has a CT number | CT number is in Hounsfield units.

Answer 158

Given information about the way the XR beams are attenuated in material HU = μtissue - μ water / μ water x 1000 μ = linear attenuation coefficient

Answer 159

``` CT number in HU Bone + 1000 - white MUscle 10-40 Water 0 Fat - 50 to - 100 Air -1000 Black ```

Answer 160

Requires the electron density to calculate dose Uses CT number/HU to calculate electron density A calibration curve defines the relationship between CT number and electron density measure on CT scanner

Answer 161

Used to change the greyscale on XR image | Out eyes can only distinguish 40-100 types of grey and the system has 256 levels

Answer 162

CT head 2mSv CXR 0.02 mSv CT abdo/pelvis 20 mSV

Answer 163

Contrast falsely elevates CT numbers | Dosimetrist can override CT numbers of TPS

Answer 164

Digitally reconstructed radiographs Images made from reconstruction of planning CT Represent same anatomy as used for planning Can be compared to kV or MV image during set up to check position.

Answer 165

DIBH - prospective gating - can monitor this automatically and set beam to only be on when in deep breath hold. Can use optical tracker on patients chest 4D CT - retrospective gating - acquire moving image of tumour. - each CT slice is timestamped and correlated with pateitn breathing phase - 4DCT volumes = MIP (maximum intensity projection), MinIP (minimum intensity projection), AIP (average intensity projection)

Answer 166

Digital Imaging and communications in medicine A standardised way of storing and transferring an image with attached patient information Uses a set file format definition and communications protocol

Answer 167

Streak artefacts- caused by highly attenuating materials eg prosthesis PArtial volume effect: when high contrast object is smaller than the slice thickness, the detector averages over the radiation intensities in z direction. Therefore size of object may be over estimated and its CT number underestimated Ghosting : caused by patient/organ movement

Answer 168

One in which the central axes of the beams lie within the same plane - if restricted to a 2D model then has to be coplanar. e.g. the arc of VMAT can be in one plane so coplanar.

Answer 169

When beams lie in a different plane Offers greater flexibility and allows improved dose distributions however often best solution in coplanar is it will minimise path length

Answer 170

Calculate dose to a 3D series of points called a dose grid Join them to make isodoses Can be used to generate DVHs

Answer 171

2D representation of 3D dose distribution for individual organs. Does not represent full isodose distribution as does NOT contain geometric information - Differential (frequency) DVH - Integral (cumulative) DVH

Answer 172

Also called frequency Fractional volume of organ recieving a dose Shows DOSE HOMOGENEITY to a structure Useful for looking at PTV as can easily see width of peak and max/min doses (narrower better)

Answer 173

Also called cumulative Fractional volume receiving that dose of greater Used more, especially for OAR Used to see the global max received by a serial organ eg spinal cord

Answer 174

3D conformal treatments are generally forward planned -> planner decides all treatment parameters (field size, beam weight, gantry angle etc) then sees what dose distribution is. Inverse planning- computer makes changes and decided if it likes it based on criteria the planner has set with the max/min acceptable doses -> computer attempts to find solutions

Answer 175

Allows us to create a curved isodose distribution | In order to do this need to modulate the radiation field in 2 dimensions -> use moving MLCs

Answer 176

While beam is on gantry rotates. Brand names: RapidArc (Varian), VMAT (Elekta), SmartArc (Pinnacle TPS) Volumetric Modulated Arc Therapy (VMAT) | We can vary: gantry rotation speed, dose rate and MLC position

Answer 177

Based on CT scanner Short 6MV waveguide spinning around patient which produces a fan beam of 6MV photons. Modulated by mini MLC Dose built in a helix with patient moving through gantry

Answer 178

Calculation algorithms already installed but no configured Done by local physics department Everytime TPS software updated the physicists have to re-check everything

Answer 179

Check CT calculating electron density correctly | Put phantom in CT with pieces of electron dense material separated by a known distance

Answer 180

Conformal plan - patient details, moves from tatoos to treatment centre correct, plan is good (covers PTV, avoids PRV), correct dose and fractionation, calculations correct, all local processes followed VMAT/IRMT - all above, plus check TPS algorithm modelling MLC movement correctly so should check on plan on linac and have software to read this and compare.

Answer 181

PANCAKE Postive Anode Negative Cathode

Answer 182

Evacuated (glass/ceramic envelope) Cathode - filament tungsten wire - electrons boiled off in thermionic emission - intensite dependent on heat/electric current Negatively charged foccussing cup - direct electrons to small are on target Anode - target - tungsten - +ve to attract electrons - high z for efficient XR production - high melting point - copper stem Filter - beam quality Collimation - hooded anode provided initial collimation (blocks stray electrons) - applicator at fixed distance

Answer 183

Significant drop off from of intensity from central axis of beam towards edge Due to HEEL EFFECT - xrs come off anode at different angles, if they come off at more of angle then have to pass through more material so more attenuated and lose more energy as heat.

Answer 184

Either a timer or an ionisation (monitor) chamber is used to control dose delivered After set time delivered voltage current turned off but most use monitor chamber

Answer 185

Historic machine when technology could only produce low energy x rays. Called teletherapy. Cobalt60 is artificially made. Produces high energy gamma rays (compared to the x ray energies being produced at the time) - half life 5 yrs (better than Radium which is the naturally occurring radioactive material previously used). - forward scattering produces relative skin sparing effect with max dose at 0.5cm - simple method, no need for high skilled maintence - can be used in developing countries - source is 2x 2cm long cylinder - large source gives large penumbra so requires primary and secondary collimator then penumbra trimmers - dose rate decreases over time and have to replace every 5 years - has radioactive source so always on - radio protection problems when decommissioning or replacing source - low energy compared to LINACs so harder to get higher dose treatment (although their spectrum is different as it is only peaks rather than a spectrum so the beam quality is higher than expected from the beam energy compared to X ray beams)

Answer 186

Electron gun = cathode = tungsten - heated = thermionic emission - thermally induced flow of electrons is produced from hot cathode Within cathode cup Anode at entrance to waveguide Electron current into waveguide - controlled by filament current and hence filament temperatures Operates in a vacuum to prevent collisions with gas molecules

Answer 187

Operates in a vacuum to prevent collisions with gas molecules - electron gun, accelerating waveguide, transmission waveguide, beam transport all occur in a vacuum - vacuum maintained using an ion pump - interlocks prevent the use of the linac if the pressure in these areas exceeds a predetermined value

Answer 188

Electrons accelerated under action of microwaves MAGNETRON - Radiofrequency oscillator, created microwaves from accelerating electrons in magnetic field - 3 Mwatts (energies up to 10MV) - physically smaller - used in low energy linacs KYLSTRON - Radiofrequency amplified - amplifies low power microwaves - 7Mwatts - mounted in insulating oil in stand Microwave generators cannot operate continuously at high power - so pulses

Answer 189

Initially electrons experience different degrees of acceleration and become bunched. Then reach speeds close to speed of light. Waveguide can be travelling or standing waveguide - either way carried like a surfer on a wave. Enter and exit - steering coils Middle - focusing coils - Solenoid - electrons tend to diverge as they travel through waveguide as repel each other - solenoid focussing coils outside waveguide, cooled using chilled water system Water cooling - accelerating waveguide, microwave generator, sterring/focussing coils, XR target all require cooling by chilled water - linac cannot be used if no chilled water supply

Answer 190

Waveguide can be inline or perpendicular to treatment head Achromatic bending magnet - all energies kept. | If perpendicular then need to bend electrons with bending magnet - this helps to focus electrons

Answer 191

- Target (tungsten - high z) - Primary collimator - Flattening filter (not required in IMRT) - Monitor chamber - Mirror - field light - Secondary collimator - Multi leaf collimators

Answer 192

3 segments - Ch1, Ch2, & segmented chamber Ch1 = primary channel, terminates the beam once the required dose has been given Ch2 = back up if Ch1 fails If that fails there is also a timer Segmented chamber- monitors beam characteristics - uniformity and symmetry. The uniformity can be used to control the gun filament electron emiussion The symmetry signal can be used to control the steering coil currents

Answer 193

Optical representation of radiation field size Light from bulb is reflected onto the secondary collimators from behind Reflecting mirror - thin foil, transparent to radiation Can aid set up Matches 50% isodose Tolerance 2mm Crosshair/wire represents centre of radiation field

Answer 194

Optical distance indicator | Bulb lights a scale that indicates distance of the patient from radiation source

Answer 195

``` 4 thick tungsten or lead alloy blocks e set of opposing jaws mounted above each other Labelled x and y jaw Can be moved to define rectangular field Move independently so can be asymmetric Usually parallel with beam to reduce transmission penumbra ```

Answer 196

Move out: - target - flattening filter Add: - scattering foil - applicator

Answer 197

Thin foils of high z material (Cu or Pb) - High z increases scattering - thin to reduce XR contamination CAUSE BREMSTRAHLUNG

Answer 198

Made of low z materials, collimates beam without generating unwanted XRs

Answer 199

2 sets of indicators - one lines up the axes of radiation beam and hence radiation beam -> field light + crosswires - 2nd indicates location of isocentre of machine -> optical distance meter at 100cm, room lasers Surface marks on patient allow them to be set up relative to isocentre

Answer 200

Isocentric rotation - allow target volume within the patient to be orientated to the maching. 6 planes: 3 rotational and 3 translational planes (forward/back, up/down, left/right, yaw, roll, clockwise/anticlockwise) Couch indexing where the patient alignment devices lock into holes on couch - allowing quick, accurate set up.

Answer 201

movement inbetween fractions e.g. anatomical changes due to weight loss H+N or pleural effusions (lung) - review and may need replanning Rectal/bladder filling - use enemas and set bladder filling protocols/ultrasounds Immobilisation e.g. shells, vac bag, tattoos to keep patient in same position Imaging used to compare to planning CT or DRR or marker seed lining up e.g. kV, cone beam CT, portal imaging (MV) | Dealt with using standard positional verification systems - mainly EPID systems using bony landmarks or markers

Answer 202

During treatment movement e.g. breathing (DIBH in breast or 4D CT in lung SABR or gating or abdominal compression in liver SABR to only allow shallow breathing), heart pumping, rectal/bladder filling (try not to take too long), patient moving (immobilisation),

Answer 203

Extremely high geometric accuracy <1mm Extremely high dose gradients - use well defined collimation systems and many beams/beamlets can spare nearby organs at risk - drops off at 25% per 1mm Only effective for small volume disease Uses Flattening Filter Free beams

Answer 204

Periphery of the target

Answer 205

Single fraction of an ablative dose, intracranially | Prescribed to a low isodose 50-85%

Answer 206

Fractionated stereotactic radiosurgery typically delivered in 3-5 doses usually intracranially and prescribed to low isodose 50-85%

Answer 207

Fractionated radiotherapy delivered extra-cranially in 3-8 fractions, usually prescribed to a higher isodose (60-80%)

Answer 208

Head frame - drilled into skull Newer ones use CBCT Uses lots of small cobalt sources - around 200 - one for each collimator Head placed into hat with tiny holes which are used as secondary collimators Patient is moved so target is at centre of all tiny beams MRI based planning Deliver in 1 day Low dose rate (so I assume takes a long time) Prescribe to 50% isodose

Answer 209

Old Linacs - add extra fixed collimator to make beam smaller and more accurate - still has flattening filter so beam more homogenous, cant get doses up really high New linacs - non-coplanar VMAT - skull is frame of reference - flattening filter free

Answer 210

6mV mini linac COlimators produce circular field sizes 5-60mm diameter Mounted on high precision robotic arm Frameless system Couples with high resolution stereoscopic kV imaging Patient position is tracked using two orthogonally placced XR camaras Non-isocentric Live images taken from each XR before each beam compared to library of DRRs created from original planning CT 4 methods of tracking- bony skull, implanted fiducial markers, bony spine, lung tumour itself Very versatile but treatment times long

Answer 211

Atomic number decreases by 2 | Mass number decreases by 4

Answer 212

Series of steps radioactive materials undergo until reach a stable state

Answer 213

Neutron in nucleus is converted into a proton and electron. Electron + anti-neutrino emitted Mass number - stays same Atomic number - increases by 1 The max energy of beta particle released is the difference in the mass between the original nucleus + post emission nucleus, remaining energy carried by anti-neutrino

Answer 214

Proton is converted into a neutron and positron Positron + neutrino emitted Mass number stays same Atomic number - decreases by 1 Once released positrons lose their kinetic energy, when most of it has gone they combine with an electron. Their combined rest mass turns into 511keV photons travelling in the other direction from the annihilation - what happens in PET!!

Answer 215

Nucleus combines with one of the orbiting electrons, converting a proton to a neutron and releasing a neutrino Mass number : same Atomic number : decreases by 1 Loss of K shell electron so another electron drops down to fill its place emitting a photon/auger electron

Answer 216

Both alpha and beta decay process may leave nucleus energetically unstable -> release gamma rays Internal conversion - in contrast to gamma decay, an energetic nucleus can sometimes release its excess energy to an orbiting electron (usually k shell) -> electron escapes -> vacancy filled by higher energy electron -> photon/auger electron

Answer 217

Becquerels 1Bq = 1 disintegration per second Or curies 1 curie = 3.7 × 10^10 Bq

Answer 218

The time for radioactive material to lose half of its activity Dependent on number of nuclei present

Answer 219

ln2 / λ λ = decay constant

Answer 220

N = N0 x e ^ - λt ``` N = amount after time t N0 = amount at start λ = decay constant t = time ```

Answer 221

Fission Neutron bombardment Charged particle bombardment

Answer 222

Occurs in a nuclear reactor Splitting of a large atom into roughly two equal parts Neutron enters nucleus, making it unstable and results in it splitting Produces Strontium 90 Tellurium 131 decays to iodine 131

Answer 223

Stable element placed in nuclear reactor Bombarded with neutrons -> rearrangement and release of gamma rays Makes beta negative decay products cobalt 60, technitium 99

Answer 224

OCcurs in a cyclotron - more expensive Makes beta positive decay products Stable element bombarded with protons or alpha particles Leads to absorption of one and ejection of one or more neutrons Makes carbon-11, fluorine 18

Answer 225

Occurs when rate of production of a radioisotope is equal to its rate of decay and so its quantity remains constant For this to occur the half life of the parent has to be greater than the half life of daughter product

Answer 226

Implanted directly into tumour

Answer 227

0.4-2Gy/hr

Answer 228

Rapid fall off dose High dose directly to tumour and relative sparing of normal tissue No need for margins to account for organ mortion Short - treatment time - reduces repopulation Can use when limited by normal tissue tolerances

Answer 229

Most common cause of litigation in oncology Very high dose close to applicators Difficult to salvage if applicators were not in right place Operator dependent Time consuming Increased radiation risk Costly

Answer 230

Developed to standardise the placement and dosimetry of iridium wire and hairpin implants Sources need to be STRAIGHT, PARALLELL and of EQUAL LENGTH with separation 5-20mm Dosimetry is calculated on the central plane midway between sources

Answer 231

Reference air kerma | On a specified date and time

Answer 232

Applicators fixed to afterloader which delivers radiation without anybody in the room

Answer 233

Source pellets and spacers are programmed and assembled in a source train. Positive air pressure system forces pellets from safe into the applicators Can be used for LDR, MDR, HDR brachy Disadvantage: all pellets have to remain in an individual catheter for same times - not flexible

Answer 234

Single stepping source system - moves the source to predetermined positions

Answer 235

Multiple channels can be connected to a variety of different applicator types Thin and flexible to go around curves Easily programmable Direct plan transfer from planning system to treatment machine

Answer 236

Back up secondary timer Automatic check of transfer tube system before source exposed Built in source position checks OPerating system check that source returned properly Back up power supply Source held in a safe so low dose around machine when not in use Manual source return in event of a complete power failure Automatic retention of treatmetn data and history in event of power failure Alarm and status code systems to alert user of faults

Answer 237

Need to calibrated on reciept Safe audits - stock checks monthly Wipe tests annually Documentation & disposal

Answer 238

Kerma rate to air, in vacuo at a reference point 1m from the source centre Units = μGy h^-1

Answer 239

The product of air kerma rate at a distance d, measured along the transverse bisector of the source and the square of the distance 1 U = 1 μGy m^2 h ^-1 = 1 cGy cm^2 h^ -1

Answer 240

GTVb - macroscopic tumour spread at time of brachy HRCTV (high risk)- whole cervix and presumed extension of tumour I- CTV (intermediate) - based on tumour prior to EBRT OAR Should report total reference air kerma, point A doses, IRU reference doses to points on rectum and bladder

Answer 241

Pre-treatment - machine function tests (interlocks, emergency equipment, audio/visual systems) - source data checks - verify date, time, source strength in planning computer Postional accuracy - verify source position of stepping source with CCTV and ruler Temporal accuracy with stopwatch Applicator integrity - inspect for any damage

Answer 242

0.38Mev (average) gamma rays Brachy wires, pins HDR brachy Iridium 192/77 = platinum 192/78 + electron + photon High specific activity

Answer 243

Neutron bombardment

Answer 244

Fission of uranium-235

Answer 245

Beta <606Kev | Gamma 364 keV

Answer 246

Thyroid, neuroendocrine tumours

Answer 247

Brachy seed - prostate | LDR

Answer 248

Characteristic XRays (from electron capture): 27.4 and 31.3keV Gamma rays: 35.5 keV Iodine 125/53 + electron = tellurium 125/52 + photon | 27.4, 31.3, 35.5 keV

Answer 249

Decay produce of Xe 125

Answer 250

Target bone mets

Answer 251

Nuclear fission

Answer 252

Beta minus decay 583 kev (mean) Degrades to Ir-89

Answer 253

28.7 years

Answer 254

Beta minus decay - emits 0.546MeV Sr-90 -> Ir -90 + electron + antineutrino Ir-90 has half life of 64hrs and decay energy of 2.27MeV

Answer 255

Neutron activation / bombardment

Answer 256

Gamma 1.17 & 1.34 Mev Cobalt60/27 = Nickel 60/28 + electron + photon + some beta decay Used in HDR afterloader and gammaknife

Answer 257

Alpha particles 5000-7500kev

Answer 258

Mets castrate resistant prostant cancer Must have tried at leas 2 systemic therapies first NO VISCERAL METS Bone scan/CT confirming osteoblastic bone mets IV injection

Answer 259

Microspheres for radioembolisation of hepatic mets

Answer 260

Beta minus decay to zirconium-90 2.28MeV - beta - travels 11mm in soft tissue Emits small percentage of bremstrahlung XR photons for post therapy imaging

Answer 261

Enclosed Inserted and removed, no radiation left inside patient Sometimes inserted and left in body in sealed iodine pellets eg prostate

Answer 262

Usually liquid | When inserted in body it disperses, patient is radioactive

Answer 263

Time taken for the biological retention of radioactivity to reduce to half its original value the time it takes for the concentration of a stable isotope to be reduced by one-half in a tissue or organ

Answer 264

Takes into account both physical and biological half life 1/te = 1/ t1/2 + 1/ tb

Answer 265

Medical Internal radiation dose (MIRD) system Limited as assumes radioactivity uniform through an organ and assumes organ sizes and shapes same as standard S values tabulated for variety of radionuclides and different source target combinations Unclear if it is clinically beneficial

Answer 266

Simple geiger counters - issues with dead time, rate may be higher than saying - use geiger counter for whole body measurement and calculate immediately after given and before 1st void -> gives calibration factor - count /known administered activity. Used for subsequent counts PET CT scans Can use the first dose to base dosimetry of next doses on if multiple doses e.g. for NET, or can do a practice run with a small dose if only one dose being given. Note in reality often don't do personalised dosimetry, just give a standard dose.

Answer 267

Well diff thyroid cancer (papillary and follicular) - adj tx post total thyroidectomy if tumour >4cm, gross extra-thyroidal extension or distand mets Dose 1.1GBq, 3.7GBq, 7.4GBq

Answer 268

Low iodine diet for 1-2 weeks prior No contrast Stop amiodarone at least 12 weeks before TSH stimulation x 2 IM injections - TSH >30

Answer 269

Acute - sialadenitis, altered taste, nausea, neck discomfort/swelling Chronic - xerostomia, altered taste, sialadenitis/lacrimal dysfunction, increased risk of second malignany, radiation induced pulmonary fibrosis

Answer 270

``` Reduce prolonged contact with friends/family Avoid public spaces SLeep away from partner Double flush toilet Wash clothes seperately ``` TFTs - suppress TSH for 5 yrs - depends on stage THyroglobulin levels 9-12 month consider stimulated TSH + US neck

Answer 271

``` 66 cycles 55kBQ/kg 4 weeks apart Alpha emitter, high LET, <1mm DNA DSBs Mimics calcium and forms complexes with bone mineral hydroxyapatite at sites of increased bone turnover Excreted in gut ``` For bony mets in prostate cancer ALP is good marker LDH sometimes useful PSA may not respond Side effects: diarrhoea, myelosuppression

Answer 272

Distance - inverse square law Shielding - linear attenuation coefficient Time - half life Justification- must be net benefit Optimisation- ALARP Dose limitation- only applied to occupational exposures

Answer 273

Biological dose - accounts for energy absorbed and LET 1Sv = 1Gy of beta/gamma 20Sv = 1 Gy of alpha

Answer 274

Radiation induced cell death DOes not occur below a tissue- specific threshold dose Severity of effect increases with dose eg death - whole body dose =5Gy Tends to occur due to direct DNA damage from ionising radiation ONce threshold dose exceed the side effects go from mild to severe

Answer 275

DNA damage by radiation No threshold dose assumed to exist Probability of effect increases linearly with dose Tends to occur due to indirect effects of ioniding radiation resultsing in free radicals. Occurs at random Can occur many years later eg further cancer USE ALARP

Answer 276

As low as reasonably practicable | Use to try to prevent stochastic effects

Answer 277

Measured in sieverts Absorbed dose x radiation weighting factor ``` Weighting factor: Photons =1 Alpha = 20 Electrons = 1 Neutrons = 5-20 ```

Answer 278

Measured in sieverts Equivalent dose x tissue weighting factor Lungs, colon, breast. bone marrow, stomach = 0.12 Gonads =0.08 Bladder, liver, oesophagus, thyroid = 0.04 Bone, salivary glands, skin = 0.01

Answer 279

2.2mSv | 6 mSv in Cornwall

Answer 280

Depends on foetal age Threshold dose of 100-200mGy or higher 1Gy can result in mental retardation/microcephaly - particularly at 8-15 weeks Increased risk of leukaemia

Answer 281

SERIOUS INCIDENT PROCESS STARTED: - 1st priority - make estimate of likely radiation dose to foetus, establish likely date of conception, estimate distance from radiation field to edge of foetus - 2nd priority- decide whether the incident needs to be reported under IRMER guidance. - currently guidance is anything >10mGy should be reported -3rd priority - carry out investigation into root cause and what can be learnt

Answer 282

Covers use of all ionising radiation (allows medical exposures covered in IRMER2017). Discusses protection of staff.

Answer 283

Health and safety of the workplace so they are aimed at protecting people who are working with radiation Regulation enforced by HSE Employer must take all necessary steps to restrict as FAR AS REASONABLE PRACTICABLE exposure to radiation Incorperates dose limits. Does NOT cover patients - covered by IRMER

Answer 284

1. Use engineering controls (prevent people from irradiating themselves) 2. Use systems of work (tell people not to irradiate themselves) 3. Use personal protective equipment (reduce any exposure)

Answer 285

Radiation Protection Advisor (RPA)- roles: - designation of controlled areas - writing and reviewing local rules - ensure adequate training - ensure RPSs appointed - monitor staff doses - assessment of clinical incidents - help design new faciclities - advice to ethics committee Radiation protection supervisor( RPS) - supervise safe use of radiation - inform RPA of potential hazards/incidents - ensure safe working practice - keep records up to date

Answer 286

``` A worker likely to recieved >3/10 of a dose limit Dose likely to exceed 6mSV/yr Must be monitored Undergo annual medical surveillance May work in controlled areas ```

Answer 287

UNlikely to recieved 3/10ths of a dose limit Can work in a controlled area under a written system of work May be monitored (not mandatory) Must be radiation protection trained

Answer 288

20mSV 6mSV 1mSV

Answer 289

Places where it is likely the dose limnit will be exceeded Dose rate >7.5microSv/hr Clearly defined and marked Can be permanent or temporary Entry to non classified workers under written system of work

Answer 290

Governs the radiation doses received by patients for diagnosis or treatment are regulated by this legislation

Answer 291

Administration of Radioactive Substances Advisory Committee Practioner licence - use radioactive substances in medicine, the practioner has to provide evidence of training - for any nuclear medicine, radionuclide or brachy tests - each licence applies for one person at that site only and lists procedures the referrer can perform - licence lasts 5 years Employer licence - states what facilities and support available - governance arrangements for IRMER - Declare procedures they plan to carry out

Answer 292

Employer - put in place standard operating procedures Referrer- provide sufficient clinical information to allow exposure to be justified Practitioner- justify each individual exposure Operator- must optimise practical aspects of exposure

Answer 293

Referrer: - provide info to allow practitioner to justify - must be in writing - referrer must be identifiable - can be referrer and practitioner but must refer to yourself in writing

Answer 294

Establish written procedures for every practive using ionising radiation Provide ongoing staff training Establish dose constraints for research programme Keep diagnostic reference levels under review Make arrangements for clinical audit Have system for reporting unintended exposures

Answer 295

Wrong patient exposed Wrong radioactive material adminstered Unintended planning or verification exposures Dose delivered to PTV/OAR is 1.1x (10%) over whole course or 1.2 x (20%) for any fraction over All total geographical misses All partial geographical misses that exceed locally defined error margin Clinically significant underdoses

Answer 296

Report to IRR99 if caused by equipment failure -> report to HSE Report to IRMER for anything else -> CQC

Answer 297

Thick concrete walls - prevent escape THicker concrete walls in direct area of beam = Primary barrier Maze - prevent radiation escaping or interlocking doors Light gate - light sensors that cut off beam Yellow light - controlled area Emergency stop buttons Beam on light Last man out button

Answer 298

All procedures that ensure the consistency of medical prescription, safe fufilment of that prescription i.e. optimal treatment of PTV, sparing normal tissues with minimal exposure to staff

Answer 299

QC procedures to ensure all processes conform to established specifications i.e. checks to establish equipment is working correctly and in tolerance quick and simple to perform

Answer 300

Image quality Image scaling Laser alignment CT number

Answer 301

Primary dosimeter- held by NPL - calorimeter for MV, electrons - - free air ionisation chamber for kV photons Secondary dosimeter - local, one for each type of dosimeter - well type for brachy - farmer for photon - parallel plate for electron Tertiary dosimeter - field dosimeters - used in practice - same as secondary but multiple of them - reference dosimetry Quaternary dosimeter - relative dosimetry e.g. TLDs, diodes

Answer 302

Every 3 years

Answer 303

When secondary dosimeter recieved back from NPL a constancy check is performed - expose to radioactive source (strontium) and can easily correct for known decay. This provides a baseline and is then done yearly before secondary dosimeter is used to calibrate tertiary dosimeters (Just in case dosimeter damaged over the year or something) Also done every 3 months for the tertiary/field chambers

Answer 304

``` Safety checks/interlocks ODI SSD measurement (2mm tolerance) Output constancy checks (2%) OPtical field size of collimator and MLC (1mm/jaw) Alignment of lasers (1mm) ```

Answer 305

``` More extensive safety/interlock tests Accuracy of mechanical scales for all degrees of freedom (gantry, collimator rotation, couch) Optical cross hair vs mechanical isocentre Optical field size test Dosimetry: output, beam energy Flatness and symmetry of beam Optical vs radiation field size MLC leaf and position Emergency off switches ```

Answer 306

MLC QA | Checks leaf positions

Answer 307

- imaging equipment and methods - tx techniques - freq and timing of imaging (for verification) - anatomical ref points - tolerances and action levels It is possible to prescribe off protocol but must justify

Answer 308

Machine set up by operators and compared to stored data - if does not agree than dose cannot go ahead.

Answer 309

Combines positional and dosimetric verification EPID = electronic portal imaging device | Exit megavoltage fluence from patient is measured by EPID

Answer 310

``` 6MV 1.5cm 10MV 2.5cm 15MV 3cm 20MV 3.5cm 25MV 5cm ```

Answer 311

Output calibration measurement ODI at different distances Pointers Wedge factor

Answer 312

Volume outside the PTV which receives dose larger than 100% of the specified PTV dose. In general considered significant only if minimum diameter exceeds 15 mm; however, if it occurs in a small organ (e.g. the eye, optic nerve, larynx), a dimension smaller than 15 mm has to be considered

Answer 313

RTOG conformity index (ideally 1 usually 1.05-1.3) = Treated volume / PTV Paddick conformity index (ideally 1 usually 0.7-0.98) = PTV that is irradiated / irradiated treatment volume X PTV that is irradiated / PTV Paddick conformity index is more useful as actually makes sure the irradiated volume overlaps with the PTV. Gradient index (want as small as possible usually 3-7, is a measure of dose gradient) = volume of patient receiving 50% of prescription isodose = volume of patient receiving prescription dose

Answer 314

``` Mean basal dose rate is the average of the minimum dose rates located between the sources inside the implanted volume. The individual minimum dose rates should be within ±10% of the average (basal dose rate), thus restricting the number of sources to be used The stated (reference) dose rate is a fixed percentage (85%) of the basal dose rate. ```

Answer 315

A free radical is an atom or molecule with an unpaired electron, regardless of its charge status, making it highly reactive with other atoms and molecules. Free radicals have halflives on the order of micro- to milliseconds. Free radicals do not necessarily possess charge.

Answer 316

Define GTV, high risk CTV (high tumour volume), intermediate risk CTV (microscopic spread) and OARs Has recommendations on dose prescriptions and OAR limits (D2cc = the minimum dose to the most irradiated 2 cubic cm volume of OAR) DVH reporting for rectum, bladder, sigmoid and small bowel Defined reporting points/volumes: point A, total reference air KERMA for whole treatment, high risk CTV and intermediate risk CTV (D98%, D90% and D50%), GTVres (D98%) = residual GTV following external radiotherapy Defined OAR volumes: bladder/rectum/sigmoid - D0.1cc, D2cc, ICRU38points); vaginal wall using surrogate posterior inferior border symphysis +/- 2cm EQD2 used to sum the EBRT and brachy in 2 Gy/fraction equivalence Use MR based brachytherapy planning for soft tissue (CT often used for applicator reconstruction then CT and MR registered)

Answer 317

Use intrauterine applicator and two vaginal ovoids. Prescribe dose to Point A (2cm superior to surface of cervical OS/ovoid and 2cm lateral to midline/IU tube) - this represents the minimum dose to malignant tissue. Point A should not receive more than 1/3 dose from vaginal ovoids - creates pear-shaped dose distribution. Calculate dose to Point B (3cm lateral to A - dose estimate to pevlic wall) Calculate dose to rectal (0.5cm posterior to vaginal wall through plane of vaginal ovoids) and bladder dose points (surface of bladder balloon). Evolved with better imaging, and variation in source dwell times. Then could add interstitial brachytherapy to bring dose out for irregular tumours but need TPS planning by GEC estro guidelines.

Answer 318

Online planning using transrectal ultrasound (patient under GA and lithotomy position) with grid against perineum in which needles inserted. Contoured prostate and rectum and urethra and CTV. Insert I125 seeds through needles. GEC ESTRO define dose rules. HDR afterloader based system can use offline planning with CT after needles inserted then insert sources after planning on CT.

Answer 319

Isocentre located at point where PTVs split. Need to ensure no divergence/overlap at the junction. e.g. can get hotspot if electrons next to photons as electrons will diverge into photon field. Main example: lateral beams in H+N above neck then A+P below neck.

Answer 320

Flatness = Dmax/Dmin Symmetry = max (D -x / D +x) Symmetry is the difference between the doses at equidistant points along the beam profile from the midline.