Overall Flashcards
Which type of waves have the longest wavelength and therefore lowest frequency?
Radio
What is the wavelength range of visible light?
700-400nm
What is 1 atomic mass unit in kg and MeV?
1.66 x10^-27 kg and 931MeV
What is 1eV in Joules?
1.6 x10^-19
What are the innermost 3 electron shells called?
K, L, M
What is the electron binding energy also called?
Ionisation energy
What is the Auger effect?
A vacancy in the inner shells (K or L) is filled by a higher energy level electron (M), which releases energy that is absorbed by an electron in a high energy level, which then gets ionised (second ejected electron is an Auger electron)
What are characteristic x-rays?
Outer-shell electrons fill a vacancy in the inner shell of an atom (which has been made from a previous ionisation), releasing X-rays in a pattern that is “characteristic” to each element
What forces work on nucleons in the nucleus?
Coulomb and strong nuclear forces
How much larger is the binding energy for a nucleon than for an electron?
1000 times
What are isotopes, isotones and isobars?
Isotopes - same no. of protons. Isotones - same no. of neutrons. Isobars - same atomic mass
1 barn is equal to what area size?
10^-28 m^2 (= 10^24 cm^2)
What does the units of barn represent?
They are a measure of reaction probability (cross sections) and it can be thought of as the size of the object that the excitation must hit in order for the process to occur
What is systematic and random error in treatment verification?
The both refer to positioning errors, where systematic is in a consistent magnitude and direction, whereas random is in varied magnitudes and directions (averaged out with large sample)
What is brachytherapy?
Using radioactive sealed sources at a short distance to treat tumours
What are the three methods of brachytherapy?
Surface moulds, interstitial (needles), intracavitary
What are the advantages of brachytherapy?
Very localised so minimises damage to surrounding tissues, short treatment times, very effective
What are the magnitude of the dimensions of a nucleus and an atom?
Atom = few 10^-10 m (angstroms)
Nucleus = few 10^-15 m (femtometres)
What principle means that only a given number of electrons can exist in the same subshell?
Pauli exclusion principle
What is the de Broglie wavelength?
Planck’s constant divided by momentum. It is the wavelength associated with a particle to consider its wave-like behaviour. Only waves with an integral number of de Broglie wavelengths around an orbit (eg electron) are allowed
What is the nuclear radius equation?
r_0 multiplied by the cube root of the mass number
Is nuclear matter density constant?
Yes
Is the nuclear mass more or less than the sum of the masses of the constituent neutrons and protons?
Less than
The nuclear binding energy is the energy released when the nucleons fuse into a nucleus and is also the energy for what?
the energy needed to separate the nucleus into its constituent parts
What is the Q value?
The energy change in any transformation process eg nuclear reaction
At what range is the nuclear force attractive and repulsive?
Attractive for distances within a few femtometres. Repulsive for distances below 1 femtometre
Is the nuclear force charge independent?
Yes
What is the decay law and its associated equation?
The rate at which nuclei decay is proportional to the number of nuclei N. Differential of N wrt to time equals minus the decay constant times N
The number of nuclei at a given time is equal to what (equation)? (same equation for activity instead of number of nuclei)
The initial number of nuclei multiplied by e to the power of minus the decay constant times time
What is the equation for activity, including the decay constant and number of nuclei?
Decay constant multiplied by number of nuclei
What is the mean lifetime equation for radioactivity?
tau = 1 over the decay constant
What is the half life equation for radioactivity?
ln 2 divided by the decay constant
Is the decay constant a characteristic of a nuclei or dependent on external factors?
Characteristic of a nuclei
What is one Becquerel?
Activity of one decay per second (the unit of radioactivity)
If you mix two or more radioactive species together, how can you work out the overall activity?
Add them
For decay chains, the differential of the number of nuclei wrt is equal to what?
The rate of decay (negative value) plus the rate of formation from the parent nuclei (positive value and uses the decay constant of the parent)
When does radioactive equilibrium occur?
When the products of a nuclear reaction decay at the rate they are produced (R = reaction rate), which happens when the time is a lot longer than the product’s half life (R = A)
What is the saturation factor in radioactivity with nuclear reactions?
1 minus e to the power of minus the decay constant times time [1 - e^(-lambda x t)]
What is the absorbed dose (D) and its units?
Energy absorbed per unit mass of material, units = gray = J/kg
What is the equivalent dose (H) and its units?
Absorbed dose multiplied by a radiation weighting factor. Units = Sieverts = J/kg
What is the effective dose (E) and its units?
The sum of equivalent doses multiplied by tissue weighting factors. Units = Sievert = J/kg
When do decay chains stop?
A stable isotope of lead is reached
What type of decay do many of the heavier nuclei in the chain decay by?
Alpha emission
When does gamma decay occur?
A nucleus is in an excited state and has too much energy to be stable, often after alpha or beta decay
What is neutron activation?
Atomic nuclei (stable target) capture free neutrons to produce an unstable product
What are the methods to create artificial radionuclides?
Neutron induced fission, neutron activation and reactions using beams of charged particles from accelerators
A typical cross section for a collision of a light particle with a medium-mass nucleus is one barn, how much is this value
10^-24 cm^2
How do you get the total cross section for a collision from the partial cross sections? (many different reactions can occur and each have their own partial cross section)
Sum the partial cross sections
After nuclear fission, all fission fragments are unstable, are they neutron or proton rich?
Neutron rich
How do neutron rich nuclei decay?
Beta minus decay
After neutron activation, how does the unstable product typically decay?
Beta minus decay
How many molecules are there in 1 mole of any substance?
Avogadro’s number
Are photon interactions stochastic by nature?
Yes
What are the two broad types of photon interactions?
Absorption processes (incoming photon loses all energy to target medium and secondary particles may be emitted) and scattering processes (photons change direction, motion, energy and momentum)
What are the main absorption processes for photons interactions?
Photoelectric effect, pair and triplet production, nuclear photo effect (normally neglected)
What are the main scattering processes for photon interactions?
Coherent (energy unchanged -elastic?) and incoherent scattering (energy changed - inelastic?)
What is the differential cross section?
The probability that a particle passing through an area of dσ (cross section) before scattering can be found within the solid angle dΩ
after scattering. (in 2D this could be an angle theta)
What is the basic premise of the photoelectric effect?
Photon interacts with a bound atomic electron (usually in K shell) and a photoelectron is emitted
What is the energy of the photoelectron after the photoelectric interaction has taken place?
The photon energy minus the binding energy
Is the photoelectric effect dominant at high or low energies?
Low
Does the photoelectric absorption cross section increase or decrease as photon energy increases?
Decreases
Where are there discontinuities in the photoelectric cross section curve?
At given binding energies of electrons in atomic shells called absorption edges (above this they can cause ionisation)
What is an absorption edge?
A sharp discontinuity in the absorption spectrum, which occur when the energy of an absorbed photon corresponds to an electronic transition or ionisation potential
What shell is the absorption edge most pronounced?
K shell (K-edge specifically named)
Why does the photoelectric effect depend on the atomic number?
It increases when the number of electrons that can participate in the process increases
What is the fluorescence yield, omega? (related to photoelectric effect)
The probability of emission of a characteristic x-ray
What is the probability of emission of an Auger electron in terms of the fluorescence yield?
1 minus the fluorescence yield (ie if it doesn’t emit a characteristic x-ray, it will emit an auger electron)
Is the fluorescence yield larger or smaller for when filling a vacancy in the K-shell compared to a shell with lower binding energy?
Larger
Does the K-fluorescence yield increase or decrease with decreasing atomic number? (K-fluorescence = probability of emission of characteristic x-ray from k-shell)
Decrease
Is the photoelectric effect probable or improbable at low photon energies, especially for materials with atomic number (like bones)?
Probable
What energy does a photon have to have in order for pair production to occur?
Exceed twice the rest mass energy of an electron = 1.02 MeV
What is pair production?
Photon above 1.02 MeV in the electric field of the nucleus turns into an electron-positron pair with any excess energy being shared as kinetic energy
What will happen after the pair production process?
Positronium formed with positron acting like nucleus with electron around it before positron annihilates with an electron when it slows down and creates two annihilation photons (511 keV each)
What is triplet production?
Pair production in the electric field of an atomic electron, which also causes the atomic electron to be ejected from the atomic shell
What is the energy threshold of triplet production>
4 times the rest mass energy of an electron = 2.04 MeV
Is the cross -section for pair and triplet production zero or non-zero below their energy thresholds?
Zero
How does the pair production cross section (above threshold) vary with photon energies and atomic number, Z?
Increases with increasing energy and approximately proportional to the square of the atomic number, Z (nuclear charge)
How does the triplet production cross section vary with nuclear charge /atomic number, Z (above energy threshold) and what does this mean?
Approximately varies with Z so it gets less important with increasing atomic number
What is a form of incoherent scattering?
Compton scattering
When the energy of the photon is significantly larger than the binding energy of an electron, how do we consider a target electron (incoherent scattering)?
Free and at rest at the time of collision
At lower photon energy scattering, what is it called when photons can scatter from individual bound electrons (1) and what is it called when it can scatter in phase from all the bound electrons together?
(1) incoherent scattering
(2) coherent scattering
What is Compton scattering?
A photon with far more energy than the binding energy of an electron is scattered through an angled theta wrt its original direction and transfer a fraction of its energy to a recoil electron
Does a photon lose any energy if it is Compton scattered in the forward direction?
No
In what direction does a photon lose the largest fraction of its energy following Compton scattering?
Backscattering (theta = 180 degrees)
In Compton scattering, does the fractional loss of energy of the photon increase or decrease as the photon energy increases (for the same scattering angle)?
Increases
In Compton scattering, the kinetic energy of the electron is given by what calculation?
The difference between the incident and scattered photon
In Compton scattering, when is the kinetic energy of the electron the largest and when is it zero?
Largest when the photon is backscattered and zero when the photon is forward scattered
In Compton scattering, is it more or less likely a higher energy photon will be scattered forwards?
More
Is Compton scattering dependent on incident photon energy?
Only weakly, it is relatively constant over the range 10-600 keV
The cross section for coherent (elastic) scattering is proportional to what and what does this mean?
The atomic number divided by the energy squared, so only occurs for low energies
Are we concerned about coherent scatter at typically used energies?
No
The cross section for the photoelectric effect is proportional to what?
Atomic number to the power of 5 divided by photon energy to the power of 3.5
What is the nuclear photoeffect?
A photon with energy exceeding the binding energy of a nucleon can be absorbed and one or more nucleons are ejected
Out of the photon interactions, which one dominates at low energies?
Photoelectric effect
Out of the photon interactions, which one dominates for increasing energies but not really high energies (intermediate energies)?
Incoherent scattering (Compton)
Out of the photon interactions, which one dominates at high energies?
Pair production
Out of the photon interactions, which one dominates for low atomic number Z for a wide range of energies (very low to several tens of MeV)?
Compton scattering
Out of the photon interactions, which one dominates for increased atomic number Z but at lower energies?
Photoelectric effect
Out of the photon interactions, which one dominate for increased atomic number Z and high energies?
Pair production
Attenuation of the beam intensity depend on what factors and with what relationship?
Linear attenuation coefficient and thickness of absorber, initial intensity times e to the power of minus mu times thickness is the intensity
What does the linear attenuation coefficient depend on?
Density of material and atomic number. (decreases with increasing photon energy)
What is the linear attenuation coefficient?
The probability per unit length for interaction
How is the linear attenuation coefficient related to the total atomic cross section?
It is equal to the total cross section multiplied by the number of target entities per unit volume
What is the mass attenuation coefficient?
Linear attenuation coefficient divided by density
What is the half-value thickness?
The thickness of absorber needed to decrease the transmitted beam by half
What is the equation for half value thickness?
ln(2) divided by the linear attenuation coefficient
What is the equation for mean free path of photons in an absorber?
1 over the linear attenuation coefficient or the half value thickness divided by ln(2)
In medical physics, even if the primary beam is a photon, what is it that causes the biological effects?
Charged particles (secondary radiation)
What are the two interaction mechanisms for heavier charged particles (»1amu) that particles loses energy?
Collision and elastic scattering
What are the three interaction mechanisms for electrons?
Collisions (inelastic scattering), radiative (bremsstrahlung) and elastic scattering
How is momentum transferred to atomic electrons as the ion passes by due to the Coulomb force?
Impulse
Do heavy charged particles lose a lot of energy in a collision with an electron and what does this mean?
No, only a small amount so the charged particle needs lots of collisions before it stops
What are delta rays?
Fast electrons that are produced when they have been transferred enough energy from a charged particle and the electrons can ionise further atoms in subsequent interactions
What are soft collisions in terms of interactions with charged particles?
Occur at a large distance where the Coulomb force field affects the atom as a whole with a small amount of energy transferred to possibly cause excitation or ionisation
Are hard or soft collisions more numerous?
Soft
What are hard collisions in terms of interactions with charged particles?
Occur at a small distance (atomic), with electrons ejected as a result with KE (delta rays)
What is the linear stopping power?
Minus the average energy loss, dE, per unit distance dx (positive overall)
What is the mass collision stopping power?
The linear stopping power of a collision divided by density
In general, does the ionisation energy (I) increase with atomic number Z?
Yes
At relativistic energies there is an increase in stopping power known as relativistic rise, why is this?
Electric field undergoes Lorentz contraction, so the moving particle is within the proximity of bound atomic electrons for less time
Is the stopping power smaller or larger for slower particles?
Larger (until it gets to relativistic speeds)
Linear energy transfer is known as restricted stopping power, what is this?
Energy transferred to material surrounding particle track from the secondary electrons (only close to particle track so not electrons with high KE)
What causes Bremsstrahlung radiation?
Electron beams interact with the strong electric field of the nucleus and get deflected and deaccelerated, emitting photons
What is the energy range of Bremsstrahlung radiation?
Nearly zero to the full kinetic energy of the incident particle (it is virtually stopped)
The intensity of bremsstrahlung radiation is proportional to what?
The nuclear charge divided by the mass of the moving particle all squared
Bremsstrahlung only accounts for a small fraction of energy losses, but why is it important to consider for shielding?
Bremsstrahlung photons generated within high-Z shielding material are more penetrating so may require low-Z primary beta particle shielding surrounded by lead
Do alpha particles and protons create bremsstrahlung radiation and why?
They do but significantly less than electrons because bremsstrahlung production is inversely proportional to the mass of the incident charged particle
What is the radiation yield?
The fraction of the initial electron energy lost to the Bremsstrahlung slowing down process
Is the radiation yield from bremsstrahlung radiation dependent or independent of incident energy and atomic number?
Linearly dependent
At high and low energies, what direction are bremsstrahlung photons directed?
At high energies they are forward focussed. At low energies, they are more isotropic
Total linear stopping power is the sum of what type of losses?
Collisional and radiative losses
For lower energies (<10 MeV), are collisional or radiative losses more important?
Collisional
What is energy loss straggling?
Fluctuations in energy loss occur about a mean value with a related width (capital gamma) of distribution
Does the rate of energy loss of a charged particle increase or decrease when its energy decreases?
Increase
What creates a Bragg peak?
A charged particle slows down and most of the energy is lost at this point just before it comes to rest
What is the Bragg curve?
Plot of energy lost along the track of a charged particle (stopping power against distance)
The distance before the Bragg peak is determined by what?
The energy of the charged particle
The range of a charged particle is given by what equation?
The integral of energy (dE) divided by stopping power (dT/dx)
What is the definition of the mean range of a charged particle?
The absorber thickness that reduces the incident intensity to half its initial value
What is the extrapolated range of a charged particle?
The distance when you extrapolate the linear portion of the end of the transmission curve to zero
Why is extrapolated range a better measure of absorber thickness to stop electrons compared to mean range?
Multiple scattering occurs so total path length is greater than linear penetration distance
For heavy charged particles, what will the fraction beam intensity (I/I_0) curve (transmission curve) look like?
Approximately one until near the end of the path when it rapidly falls to zero
When does elastic scattering happen for charged particles?
Charged particle passes close to atomic nucleus (less than atomic radius) and is scattered by localised force field
What is the units for linear stopping power and mass stopping power?
Linear stopping power: MeV/cm
Mass stopping power: MeV.cm^2/g
What is brachytherapy not usually good for and why?
Large tumours at depth as hard to get a consistent dose
What are the properties of an ideal brachytherapy source?
Appropriate energy range and half life, no charged particle emission or screened easily, no gaseous decay, high specific activity, non-toxic, mechanically stable, easily moulded and sterilisable
what energy range is brachytherapy and why?
0.2 - 0.4 MeV as don’t want radiation to travel too far
What is the decay mode for iridium-192 in brachytherapy?
Beta minus decay (95%) and small amount of gamma (5%)
What is afterloading in brachytherapy?
The ability to deliver a radioactive source into a catheter or applicator after it has been correctly positioned for treatment
Although not used anymore, what is manual afterloading in brachytherapy?
The applicators are positioned, the radiation source is manually loaded into the applicator
How are dose distributions in brachytherapy obtained?
Dwelling the source at different positions at different times
What design features are needed for a treatment room with a HDR brachytherapy source?
Thick concrete walls with a maze entrance, control desk outside room, radiation warning lights, interlocked barrier at room entrance, audible alarm when source exposed
Why is it important to do a source calibration with two physicists and two equipment sets of a new HDR brachytherapy source?
It can be a major source of systematic error
Is brachytherapy normally done as an outpatient or inpatient procedure?
Outpatient
What imaging modality is often used in brachytherapy with the applicator in?
MRI (or CT)
What is image guided brachytherapy (IGBT)? (recent development)
Uses 3D image data set to produce individualised plans
Why is the use of radiation detectors important in radiotherapy?
Accurate dose determination for ensure safe and effective treatment as a few percent difference can lead to overdose side effect or underdosage tumour control failure
What characteristics should radiotherapy radiation detectors have?
Small volume (good spatial resolution), high SNR, high sensitivity, long term stability, low energy dependence, doesn’t significantly impact beam, practical
What are some radiation detector types?
Ionisation chambers, diodes, TLD, MOSFETs, OSL (optically stimulated luminescence), gafchromic film (a type of radiochromic film) and portal imager
What structures of ionisation chamber are there?
Cylindrical (like Farmer chambers) or plane parallel plate (like ROOS) or large volume
Where are the electrodes in a Farmer chamber?
One central electrode in the middle and the outer electrode looks like the casing
What type of radiation detector is better for photon beams and which are better for electron beams?
Photon beams: cylindrical chambers (eg farmer)
Electron beams: parallel plate (eg ROOS)
When are large volume ionisation chambers used?
Low dose rates, like radiation safety. Allows increased signal but decreased spatial resolution
What are diode radiation detectors?
They are small volume solid state semiconductor detectors
What does MOSFET stand for?
Metal Oxide Semiconductor Field Effect Transistor
What is the difference between radiochromic film (eg gafchromic film) and xray film?
No developing process is required for radiochromic film (but can be read out)
Can radiochromic film be used to measure absolute dosimetry measurements and when is it usually used?
Yes, it is typically used to measure the 2D exit dose by being place behind the patient
What are Electronic Portal Imaging Devices (EPIDs) and where are they used?
Digital x-ray imaging detectors that use the MV beam for setup and dose verification for radiotherapy patients (replacing film)
What is the voltage range of orthovoltage xrays?
150 - 500 kVp
In an x-ray tube, what could be around the cathode filament and why?
Cathode shield to focus the electrons onto target
What is the purpose of cooling fins/fans behind the target of an xray tube?
Helps to dissipate heat from the target
Why may there be a tungsten radiation shield that acts as an anode hood around the target in an x-ray tube?
Absorbs stray x-rays and secondary electrons
What is the target made of in x-ray tube typically?
Tungsten bonded to copper stem
What is the atomic number of tungsten?
74
What does the magnitude of the anode heel effect depend on?
kV and angle of target
What is the heel effect?
The beam profile is asymmetric in the anode-cathode direction due to self absorption in the anode
What 3 things are outcomes of the electron interacting with the target in an x-ray tube?
Heat, bremsstrahlung radiation and characteristic radiation
How is the radiation field shaped in orthovoltage x-ray treatments?
Applicators attached to the x-ray tube to collimate the emitted beam then lead cut outs to further shape the beam
What is orthovoltage radiotherapy used for?
Superficial small cancers, like skin cancers, or non-malignant skin conditions
What does it mean that linacs usually have isocentric mounting?
The couch, collimator and gantry rotate around a fixed point called the isocentre
What is the basic principle of linacs to produce photons?
Electrons accelerated in a linear waveguide, collide with a heavy target and produce bremsstrahlung x-rays, which are collimated into a useful beam
What is the source of the electrons in a linac?
An electron gun, which has a heated filament
What does the pulse modulator do in a linac?
It syncs the signal to the magnetron and electron gun so that they work together
What does the magnetron (or klystron) do in a linac?
It produces microwaves to power the accelerating waveguide
How does the waveguide in a linac accelerate electrons?
Electrons are accelerated in a cavity using an oscillating electric field supplied by the magnetron (or klystron)
What does the waveguide in a linac look like?
An evacuated tube separated by doughnut shaped discs into several cavities
Why is the first section of a waveguide in a linac called a buncher?
The cavity sizes increase as electrons take less time to traverse the cavity and the electrons bunch up as they get funnelled together
Beyond the buncher section in a linac waveguide, the electrons approach the speed of light, what is this section called and what happen to the energy increase?
Relativistic section and energy gains mainly come from relativistic mass increase
In the linac waveguide, instead of using electrical potentials, how are electric fields created?
Microwave EM waves
Why do electrons bunch up in the buncher section of linac waveguides?
The acceleration the electrons experience depend on the electric field at that point and some get accelerated more than other and some decelerate so they bunch together
What is the difference between a 270 degree and a 90 degree bend in a linac?
270 degrees have a higher isocentre and better control of exit position
What is in a linac treatment head?
Several retractable x-ray targets (depends on photon energy needed), flattening filters, scattering foil (electron beams), primary collimator, secondary collimator, dual transmission ion chamber, field light, wedges and multileaf collimator (MLC)
What is the flattening filter in linacs?
It has more attenuation in the middle to flatten the photon beam (not electrons)
What is the dual transmission ionisation chamber in the linac treatment head?
Two separately sealed ion chambers with independent power supplies and provides monitoring of beam symmetry, flatness, dose and dose rate
What are FFF energies in linacs?
Flattening filter free (removes flattening filter in the treatment head) used for 6 and 10 MV
Why would you want to use FFF energies?
Softer beam, non-flat beam may be useful, much higher dose rates available so can treat quicker
What is IGRT?
Image-guided radiation therapy
Positioning errors can be divided into what two types?
Systematic (consistent magnitude and direction) and random (varies)
For radiotherapy treatments, do random or systematic errors matter more?
Random errors should average out over the course of a treatment but treatments will less fractions don’t follow this
What are sources of uncertainty in positioning for radiotherapy?
Organ motion, setup error, intrafraction motion, geometric accuracy of position & treatment systems, delineation (outlining accuracy)
What is immobilisation for in radiotherapy?
Keep patient in reproducible position and should be comfortable as this will reduce intrafraction patient movement
What are some ways to immobilise internal anatomy for radiotherapy treatments?
Rectal emptying and bladder filling protocols (full bladder pushes bowel out the way), abdominal compression, breath holds (DIBH = Deep Inspiration Breath Hold)
How do radiotherapy treatments work with deep inspiration breath holds do ?
Only treating at one point in the respiratory cycle to reduce tumour motion
How is it ensured patients are positioned accurately for radiotherapy treatments?
In the planning scan, they will take a CT reference image and place tattoos. For every treatment, they will use tattoos, lasers and planned moves to isocentre and then image this position for checking
What is a Digitally Reconstructed Radiograph (DRR)?
Creating a 2D x-ray image t a certain angle and position using a CT scan
For monitoring accuracy in radiotherapy treatments, the image acquired may be 2D. What reference image is used to compare against it?
A digitally reconstructed radiograph (DRR) of the planning CT scan
What imaging methods are used for monitoring accuracy in radiotherapy treatments?
Megavoltage imaging, kilovoltage imaging (planar radiographs and CBCT), and 4D CBCT
What is megavoltage portal imaging in radiotherapy?
Uses the MV beam from the linac itself using an electronic portal imaging device (EPID)
What is an advantage and disadvantage of using MV portal imaging in radiotherapy?
Advantage: imaging and treatment isocentre align and can use MLC to show treatment aperture
Disadvantage: MV energy means contrast between bone and tissue is not great
How are kV images obtained for radiotherapy?
It requires a separate x-ray source to linac that can be mounted on the linac or in the room.
What are the benefits and downsides of linac mounted kV imagers?
They generally acquire an anterior and lateral image and radiographers are experience at matching these images. But its limited to the couch being at 0 degrees
What are the benefits and downsides of room mounted kV imagers for radiotherapy?
Images can be acquired at most couch/gantry positions and low dose so extra images more justifiable. But hard to analyse oblique angles
What are fiducial markers in radiotherapy and what are they used for?
Tiny metal implants (seeds and clips) placed with a fine needle. This is done if bones are not a good surrogate for tumour position to guide treatment.
For implanting fiducial markers in radiotherapy, what should be ensured first?
Patient is well enough for insertion, enough markers to see rotation of tumour (>=3), markers distributed well in tumour, markers don’t migrate
How does cone beam CT (CBCT) work in radiotherapy?
kV source and detector rotate around patient to acquire a low quality, limited field of view CT
When is it useful to use CBCT?
Soft tissue detail when other forms are impractical (bone or fiducial markers). Tumour growth/shrinkage, weightloss, change in rectal bladder filling, lung issues (eg collapse)
What are the limitations of CBCT?
High dose, limited field of view, poor image quality compared to normal CT, cannot calculate dose directly, slow scan time
What is 4D image guided radiotherapy (IGRT)?
Moving 3D image (ie image all parts of breathing cycle and play back as a movie)
What is the benefits of using 4D image guided radiotherapy (IGRT)?
Reduces uncertainty of tumour placement due to breathing motion, so can use smaller margins and avoid excess tissue treated
What is the internal tumour volume (ITV) which can be measured using a 4DCT?
All areas where the tumour is at some point during the breathing cycle
What is the problems with gating radiotherapy treatments?
Takes more time, stopping and starting of beam can introduce errors, need to be sure markers accurately measure tumour motion
What can the MV imaging panel in radiotherapy measure as well as position?
Dose (so it can be used to compare with predicted dose distributions)
If QC is the measurements we make on a linac for testing regularly, what is QA?
The whole process that determines what measurements we should make
What does PDCA/PDSA cycles stand for?
Plan, Do, Check/Study, Act
Can you use PDCA cycles when considering equipment and QA?
Yes
What is the difference between accuracy and precision?
Accuracy is the closeness between the result and the true value, whereas precision is about repeatability and reproducibility
What is precision limited by?
Resolution of the instrument
What are type A and type B errors/uncertainties and can they be measured?
Type A = random and can be measured
Type B = systematic and have to be estimated
What limits the accuracy that is achievable in radiotherapy?
Calibration, determination of dose in non-standard conditions, treatment planning and treatment delivery
Are calibration protocols for radiotherapy now based on air kerma or water and has this reduced the uncertainty?
Water and yes (used to be air kerma)
What risk analysis technique is used for planning QA of new techniques in radiotherapy?
Failure Mode and Effects Analysis (FMEA) of each stage of the process
What methods are used as major error prevention methods in radiotherapy?
Identify patients properly, independent checks, QA with in vivo dosimetry
What is the Heinrich triangle?
Major incidents in the top third, then minor incidents then near misses. If you can reduce the number of near misses, we reduce the top parts of the triangle
What type of reporting system should we have to learn from mistakes?
No fault reporting systems
What are the two aspects of QA in radiotherapy?
Maintaining accuracy for all patients and avoiding errors
What is the percentage depth dose?
The absorbed dose from a radiation beam at a certain point divided by the dose at the depth of dose maximum
With photon beams the energy decreases with depth, except with what region?
The build up region at the start, where the dose increases initially before decreasing again
After the maximum dose depth from a photon beam, what reduces the dose?
Inverse square law, attenuation and scattering
What is the tissue maximum ratio (TMR) and tissue phantom ratio (TPR)?
The ratio of the dose at a depth of interest to the dose at a reference depth at the same point, ie same distance from source to point (TMR: depth of dose maximum, TPR: any depth, usually 10cm)
Is the tissue maximum ratio (TMR) a special case of the tissue phantom ratio(TPR)?
Yes
What is the beam profile?
The variation of dose across the beam at a constant depth
What is the edge of the beam profile called when it curves?
Penumbra region
What is the penumbra width?
The difference in mm between the dose at 80% and 20% (around 6 mm)
At what percentage on a normalised beam profile graph is the geometric width of the beam?
50%
What are the two inputs to create a radiotherapy treatment plan?
CT planning scan with outlined tumours and OARS and create a model of the beam data in the planning system
After a treatment plan is created, what are the next steps?
Obtain plan approval by clinical, check plan dosimetry then transfer plan parameters to treatment machine
What are the two types of images that will be used to visualise the tumour for radiotherapy?
Diagnostic images as part of the patient referral (CT, MR or PET) and the planning image (usually CT)
The CT planning scanner is calibrated with a calibration curve (HURED) to relate Hounsfield units to what?
Relative Electron Density
What are the characteristics of a planning CT scanner in radiotherapy?
Flat couch top, wide bore, couch adapted to take immobilisation aids, ceiling and wall mounted lasers
What is the relative Hounsfield units of water, air and bone?
Water: 0, air: -1000 and bone: 700
How is the calibration curve made for relating the relative Hounsfield units from a CT image to relative electron density?
A phantom with inserts that replicate different types of tissue with known electron density
What is the HURED (Hounsfield Units to Relative Electron Density) curve only valid for?
Human tissue, eg not implants like hip[ replacements
What is delineating volumes in radiotherapy?
Outlining tumours and OARs
Which part of the treatment planning process has the greatest uncertainty and why?
Delineating volumes because not all tumours are visible on CT or not to its full extent
Why are margins required in outlining volumes in radiotherapy?
We give the treatment over several fractions so there is uncertainty in maintaining a reproducible position
What is the gross target volume (GTV)?
The gross palpable or radiologically visible extent and location of malignant growth
What is the clinical target volume (CTV)?
This is a tissue volume encompassing the GTV but with an additional margin (CTV > GTV)
What does the CTV account for that is not considered in the GTV?
Any microscopic extension of the primary tumour or regional lymph node spread to ensure it is removed completely
What is the planning target volume (PTV)?
A geometrical concept to allow for uncertainties in the planning and treatment processes (including organ motion and positioning) by adding a safety margin (PTV > CTV)
What is the result of combining percentage depth dose and beam profile shape?
Isodoses (line of relative dose - relative to max dose)
Why are multiple beams from different directions used?
A single beam for a deep tumour will give an unacceptable surface dose to the patient
What is the isocentric technique in radiotherapy for multiple beams?
All beams have a common focus point (isocentre), and the centre of the target volume is place at the machine isocentre (ie distance to the target point is kept constant for all beams)
What is the alternative to isocentric setup (= constant source-axis distance = SAD technique)?
Constant Source-surface distance = SSD technique
Does the SAD/isocentric or SSD technique require adjustment of patient setup when turning the gantry to the next field?
SSD technique
What are non-coplanar beams?
Beams that are not on the same plane and cannot interact with each other
How can you use the isocentric approach with non-coplanar beams?
Rotate the couch isocentrically so that it turns around an axis passing through the machine isocentre
Does the SSDs stay the same or vary when using isocentric plans?
They vary at beams of different angles
What is the normal fixed SSD that beams are usually measured at?
100 cm
When are fixed SSD treatments used today? (mostly use isocentric/SAD technique)
Occasionally for simple parallel opposed plans
What is beam weighting?
It specifies the extent to which different beams contribute to the overall dose-distribution
What is isocentric weighting?
It defines the beam weights as the fractional contribution to total dose at the isocentre
How are treatment plans normalised to enable plans to be correctly evaluated?
Isodoses are normalised to a reference point, which is typically in the centre of the PTV, where the dose is not changing and can be reliably calculated
What are wedged filters (wedges) in the beam in radiotherapy?
It is an angled piece of high density material that shapes the beam to create a gradient in radiation intensity with a thick and thin end
What is a multi-leaf collimator (MLC) in radiotherapy?
It is a device in the linac treatment head made of individual leaves of high atomic number material that can move independently to shape the beam
What should a QC specification have for radiotherapy?
What characteristic is to be tested and what test, what equipment is required for the test and how is it set up, how often is it done, who is able to do the test and who is responsible for reviewing the results
What levels are there for QC test results?
Normal behaviour, action level and suspension level
What happens if a QC test breaches an action level?
Equipment can still be used but schedule recalibration or investigation
What happens if a QC test breaches a suspension level?
Stop using equipment and have an urgent recalibration or investigation. Don’t use until sorted
What do the choice of action and suspension levels depend on?
What the equipment is used for, manufacturers recommendation, limitations, of measuring system, risk of recalibration
What are some broad groups of QC tests for radiotherapy?
Safety, treatment, imaging, mechanical, connectivity (between systems)
What is included in the safety QC tests on a linac?
Interlocks, touch-guards, warning lights/alarms, electrical and mechanical
What is included in the treatment QC tests on a linac?
Output, output linearity, energy, uniformity (flatness and symmetry)
What is included in the imaging QC tests on a linac?
kV image quality, CBCT dose, CBCT HU, imaging isocentre calibration
What is included in the mechanical QC tests on a linac?
Room lasers, light field, accessories, scales and displays, and robotic couch calibration
What is included in the connectivity QC tests on a linac?
Check links between different systems. eg data transfer and security, plan loading, record saving, image storage and gating connectivity
What is the gold standard code of practice method of measuring output of a linac?
Water output (true value of absolute radiation dose)
What is the easy method of output measurements of a linac that may be used for constancy checks?
Using a constancy device (box on bed or EPID)
How frequently are output constancy checks completed on linac machines?
Daily
What factors determine QC test frequency?
Likelihood and consequence of fault, detectability of fault, manufacturers recommendations, published standards, legal requirements, resource availability
