Test 3 Flashcards
Process of an atom acquiring a positive or negative charge; radiation strips an electron from a neutral atom to create a negative ion
Ionization
Occurs when a charged particle such an electron, proton, or alpha particle collides with matter to produce a charged particle; interacts with tissue
Directly ionizing
Occurs when an uncharged particle or radiation such as a photon or neutron liberates a directly ionizing particle when they interact with matter; near tissue and creates chain reaction
Indirectly ionizing
Number of photons that pass through an imaginary cross section of a sphere; flow rate of beam, how much radiation is going through
Fluence
Fluence per unit time
Fluence rate/flux density
If a photon beam is monoenergetic, attenuation will occur in an __________ manner; when beams are polyenergetic, then the beam is _________
Exponential; hardened
Low energy photons are filtered out and the beam therefore acquires a higher average energy than before
Beam hardening
To measure a beam’s transmission through an absorber, the measurement must be done with scattered photons not measured
Good geometry
5 types of interactions that cause attenuation of a photon beam by an absorbing material
Coherent scattering Photoelectric effect Compton effect Pair production Photodisintegration
Each interaction has its own attenuation coefficient (μ/ρ) which varies with _________ and ____________
Photon energy; atomic number (Z)
A photon passes near an electron and sets it into an oscillation; the oscillating electron then re-radiates the energy at the same frequency as the incident photon
Scattered x-rays have same wavelength as the incident beam in coherent scatter and equal energy
Coherent occurs at low photon energy and Z
Energy range less than 10 keV
Coherent scatter
Classical or Rayleigh scattering
The photon interacts with an atom and ejects on of the orbital electrons; the photon gives 100% of its energy to the ejected electron
A domino effect may then occur with the discrete energies being emitted and even giving off Auger electrons
Interacts with inner electron and causes cascade
Increased photon energy = less of a refraction angle of ejected electron; decreased photon energy = higher refraction angle of ejected electron
Probability increases with an increase in atomic number (Z) and decreases with energy
Probability of coherent scattering is inversely proportional to the 4th power of the wavelength
Energy range of 60-90 KeV (diagnostic)
Photoelectric effect
Photoelectric effect Z dependance
(𝑍^3/𝐸^3 )
An incoming photon hits an outer orbital electron & not all energy is transferred. This results in an ejected electron and a weaker photon.
Binding energy of electron less than incoming energy of photon
This is the most important/dominant reaction in radiation therapy
More forward peaked: low energy scatter all over, high energy peaks forward; increase energy = more forward peaked radiation
Radiation is scattered at right angles and backward
Independent of Z, dependent on electron density; electron density decreases slowly with atomic number
Energy range of 25 keV-10 MeV
Compton effect
An incoming photon interacts with an electron and gives up all of its energy in creating a positron and negatron
Charge conserved = neutral
Positron loses energy and combines with a free electron to give rise to two annihilation photons with 0.511 MeV each (annihilation reaction)
Mass goes back into energy
Pair production threshold required for occurrence is over 1.02 MeV
Probability increases with Z (twice the mass of a resting electron = 0.511 MeV)
Probability of pair production increases with Z (Z^2)
Pair production
Energy given to each positron and electron during pair production
(hν - 1.02 MeV)/2
Kinetic energy loss per unit path (length)
Electron stopping power (MeV/cm)
Max range of electrons
10 cm
Electrons lose ____ MeV per cm
2 MeV
Neutrons interact by 2 processes
Recoiling protons from hydrogen and recoiling heavy nuclei from other elements
Nuclear disintegrations
Energy range of neutrons
Above 10 MV
Represents dose versus depth
Dose-depth curve
Where dose rises from skin surface to its maximum value
Fluence is maxed-out at surface and declines as the depth increases, however attenuation (absorption) is what deposits dose
With low attenuation at surface, skin sparing occurs
Build-up region
Depth at which dose is maximum; maximum dose as a percentage of beam attenuation
Where electronic equilibrium occurs
As photons move into a medium, they set electrons in motion; electrons then deposit dose along their tracks
Increases with energy
Surface dose occurs before from backscatter electrons and contamination
Low energies continue to interact and go off in interaction
Dmax
Region beyond where dose falls steeply and almost linearly; photons drops exponentially
Fall-off region
Important reaction due to unwanted neutron production
More common in high Z materials; not in the patient but the machine head
Occurs after 10 MV
Gamma bombards element and gives off neutron
Photon comes in and neutron goes out
A/ZX + y -> A - 1/Z + 1/0n
Photonuclear reactions (y,n)
Sum of mass attenuation coefficient for photoelectric, Compton, and pair production
At intermediate energies where compton (relies on electron density) is dominant, this is slightly less for lead than water because lead has a somewhat lower number of electrons/gram than water
Total mass attenuation coefficient (u/p)
2 interactions between particles
Elastic collision
Inelastic collision
Total kinetic energy of all particles is the same before and after collision
Elastic collision
Some energy lost and goes to excitation, ionization, or brems
Inelastic collision
2 means by which electrons lose energy
Collisional
Radiative
Electron interacts with another electron
Collisional
Electron interacts with a positively charged nucleus
Radiative
In head of machine, electrons are hitting hardware and creating x-rays themselves (dose readings deeper than electron range)
Electrons produced in head of treatment machine as photons scatter off the high-density metal components in the head; also some electrons will be produced by interactions of the photons in the air between the source and patient
Electron contamination
Distance a charged particles travels before coming to rest
Range
Heavy charged particles must have superimposed polyenergetic beams for sufficient tumor coverage
Spread-out Bragg peak
Heavy charged particles have a sharp peak in energy deposition near the end of the track
Bragg peak
TD5/5 of skin
5000 cGy per 100 cm^2
Amount of x or y-radiation to produce reddening of skin
Skin erythema dose (SED)
Amount of ionization in air, produced by photons; measurement of the ability of a photon beam to ionize air
Total charge of ions of one sign produced in air when electrons (negatrons and positrons) are liberated by photons in air of mass
Only valid in air for photons up to 3 MeV, only for x- and y-rays
Exposure (X)
Traditional and SI unit of exposure
Traditional: Roentgen (R)
SI: Coulomb/kg of air
Unit of charge
Coulomb
Establishes standards for radiation, quantify volume of tumors and how they’re treated
International Commission on Radiation Units and Measurements (ICRU)
Total number of particles entering a sphere of small cross-sectional area; flow rate of photons
Units per area (m^-1 or cm^-1)
Fluence
Sum or initial kinetic energies of all charged particles liberated by uncharged ionizing particles in a mass of material
Units: J/kg
Not all energy absorbed here, some may be radiated away by Brems emission from the charged particles and the charged particles move off to a different locality
Kinetic energy released per unit mass in a medium at a specified point
KERMA
Total energy absorbed in mass (m) of material from indirectly or directly ionizing radiation
Units: J/kg, SI = Gy, traditional = rad
Energy actually absorbed in the medium at a specific location; kinetic energy being stopped
Dose in medium that describes radiation quality
Applies to all energies, radiation types, and materials
Measure of biological significant effects produced by ionizing radiation
Absorbed dose
X-ray photon interacts with an outer shell electron with sufficient energy to eject it from orbit and alter its own path
Scatter
2 factors that cause surface dose
Backscatter
Electron contamination
As electrons move through the medium, they can be scattered through large angles and some electrons can be scattered back toward, and reach, the surface
Backscatter
Dose delivered at center of a sphere of a medium which is just large enough to have electronic equilibrium at its center
Dose in free space (Dfs)
Most accurate method to compute energy deposition in matter
“Gold standard” for evaluating the detailed consequences of the interaction of radiation with matter
Tracks particle history for each interaction
Monte Carlo Algorithm
Average energy deposited per unit path length to a medium by ionizing radiation as it passes through that medium; deciding factor for quality factor
Linear energy transfer (LET)
4 main applications of radiation measurement instruments
Radiation machine calibration
Survey work
Personnel monitoring
In vivo patient measurements
Mean energy to produce ion pair
33.97 eV/ion pair
Exposure to dose conversion ratio that depends on medium and beam energy (will drastically change for low energy)
Fmed
Exposure formula
X = M(Nx)(Ctp)(Cst)(Cion)
X = exposure M = chamber reading by electrometer Nx = chamber exposure calibration factor given by calibration lab Ctp = correction for temperature and pressure Cst = correction for stem leakage Cion = correction for ion recombination
Chamber exposure calibration factor given by calibration lab; how much output electrometer has
Nx
Corrects for ions that recombine before being measured
A loss of charge occurs as the ions that are created recombine with each other and never reach the collecting electrode
Always 1 or more; typically less than about 2%, less than or about 1.02
Reciprocal of the collection efficiency = 1/f
Correction for ion recombination (Cion)
Standard pressure
760 mmHg
Ctp formula
Ctp = (760/P)(273+C/295)
Boiling points of fahrenheit and celsius
212 F
100 C
Fahrenheit to celsius formula
C = (F-32)(5/9)
Celsius to fahrenheit formula
F = (C x 9/5) + 32
Dose delivered at center of a sphere of a medium which is just large enough to have electronic equilibrium at its center
Sphere surrounded by air (in free space); sphere no larger than minimum diameter for electronic equilibrium
Dose in free space (Dfs)
Dose formula
D = Fmed (X) (Aeq)
Very accurate measurement of radiation amount emitted by radiation producing device, not sensitive as radiation level is already very high
Radiation machine calibration instruments
Detect and provide rough measure of radiation levels in environment; need to be sensitive but not highly accurate (doesn’t measure
Used for detecting and locating radiation contamination
Survey work
Track radiation worker doses
Need to be sensitive and measure small amounts of radiation
Must be able to measure cumulative radiation exposure
Personnel monitoring
Monitor amount of radiation patients receive during treatment
In vivo patient measurements
3 categories of radiation detectors classified according to medium used for detection
Gas ionization detectors
Solid-state detectors
Liquid dosimeters
3 gas ionization detectors
Ion chambers
Proportional counters
Gieger-Muller (GM) counters
6 solid-state detectors
Thermoluminescent dosimeters (TLDs) Film Diodes Metal oxide semiconductor-field effect transistors (MOSFETs) Polymer gel Scintillation
2 liquid dosimeters
Calorimeters
Chemical
Collect charge; very accurate measurement of amount of radiation
Beam calibration and survey meters
Ionization chamber
When radiation passes through the gas between the charged electrodes, it produces ion pairs, which are attracted to the plates having charge of opposite sign
As the charge is collected, it’s registered as a current on the ammeter
Get chamber reading by electrometer
Rely on ionization of gas producing ion pairs
Without radiation current doesn’t flow due to distance between electrodes; with radiation ion tracks are produced
Gas ionization chamber/detector
Amount of ionization in air is dependent upon photon energy; increase energy = _______ ionization rate
Increase
Ion chambers collect current, measures charge
Electrometer
Some ions produced in ion chamber collection region are lost and some that are produced outside the collection region are collected; when these values are equal
Scatter in = scatter out
Electronic equilibrium
Dmax
Measures exposure, absolute dosimeter
Primary standard used for calibration of secondary instruments used in the field
Used in National Standard Laboratories (NIST); uses is Accredited Dosimetry Calibration Laboratories (ADCL)
Very delicate
Practical in the range 10-300 keV; above about 3 MeV (at best) the plate separation must be impractically large
Annual exposure
X-rays enter chamber and ions are produced all along the volume occupied by the beam
Electrons produced by the photons move through the air creating tracks of ionization and charge collected from throughout the entire collecting volume by the electrode (collecting volume)
Free-air ion chamber
Number of ions generated by fixed amount of radiation will depend on the mass of the air inside the ion chamber; the greater the mass of air, the ____ ions produced
More
An air cavity with a “shell” of air surrounding it which is sufficiently thick to provide charged particle equilibrium (want to measure at Dmax or chance for error) inside the air cavity
Cavity ionization chamber
3 majors types of cavity ion chambers
Thimble
Flat-cavity/plane parallel
Well-ionization for brachytherapy
As ion chambers are too delicate and bulky for everyday use, these smaller models are used; must behave like a free-air chamber
Voltage placed between inner thimble wall and central electrode
If the central electrode is made positive, any negative charge produced in the cavity by the passage of ionizing radiation will be attracted to it
It will then flow out through the wire attached to the central electrode and be counted by the electrometer
Wall is solid but air equivalent (Z = 7.6)
Graphite or aluminum central electrode, inner wall, plastic covered
1 mm or less thickness
Major limitation is surface dose
Collecting volume: 0.1-1 cm^3
Calibrated every two years
Thimble chamber
3 ideal chamber characteristics
Have a suitable volume to allow measurement for expected range of exposures
Minimal variation in sensitivity or exposure calibration factor over wide energy range, with incident radiation direction, stem leakage, and ion recombination losses
Should be calibrated for exposure against a standard instrument for all radiation qualities for which exposure will be measured
Thimble chamber to provide stable and reliable secondary standard for photons for all energies in therapeutic range; orthoganal
Commonly used for external beam calibration, often used in water tank for beam calibration purposes
Wall made of pure graphite, central electrode of pure aluminum, and insulator of polytrichlorofluoroethylene
Collecting volume is 0.6 cm^3
Buildup cap
Dose changes rapidly with depth; ex: buildup region, electron beams
Large inner cavity can disturb low energy electron beams and therefore provide inaccurate readings
Farmer chamber
3 electrodes in well-guarded ion chamber of farmer chamber
Central/collector electrode
Guard electrode
Thimble wall
Collects ionization charge and delivers current to the charge measuring device; 1 mm aluminum rod
Central/collector electrode
2 purposes of guard electrode
Prevent leakage current from collector electrode
Define ion collecting volume
Ground potential and kept at same potential as collector electrode (300 V)
Thimble wall
When irradiated, the chamber stem and cable may get ionization that is measured by the chamber; can be 1-10%
Corrects reading for the falsely created charges collected
Stem effect/leakage
Correction for stem leakage (Cst)
Increase energy = _________ stem effect
Increased
Easily measures surface dose; used in clinic to measure superficial dose
Have two thin, closely spaced parallel collecting plates/electrodes; can be sealed or unsealed
Electrode space is fixed at 2 mm
Solves problems with farmer chambers: hard to measure dose in buildup region because of large gradient (rapidly changing)
Linear accelerator has two to have a double-check in case of failure
One electrode is thin window where radiation enters 0.01-0.03 mm thick
Collecting diameter of 5 mm provides high spatial resolution in beam direction
Parallel plane chamber/pancake
Sensitive instrument designed to detect the presence of radiation
Used in brachytherapy (ex: check if source fell out of patient)
Don’t use to measure patient dose, MV, calibration, etc. just to detect presence
Survey meter ion chamber
1 Roentgen (R) = ______ rad in air
0.876 rad in air
Used for survey meters
Very sensitive gas detectors, not very accurate and doesn’t measure amount
Good for locating lost radioactive sources of finding radioactive contamination; ex: survey brachytherapy room
Designed for maximum gas amplification; often has audible clicks that correspond to each count
Typical range of 0.01 mR/hr - 1 R/h
Gieger-Muller (GM) counters
1 rad = ____ cGy = ____ ergs/g
1 cGy = 1 ergs/g
1 rad = ____ J/kg = ____ Gy
1x10^-2 J/kg = 0.01 Gy
Ratio of energy fluence at center of equilibrium mass to the same point in free air space
Increases with decrease in energy
Transmission factor (Aeq)
6 radiation dosimeters
Calorimetry Chemical dosimetry Solid state methods Silicon diodes Radiographic film Radiochromic film
Operates under the principle that almost all energy deposited in a medium of water will appear as heat
Generally only found in standards labs such as National Institute of Standards and Technology (NIST)
Insulated container used to measure small amount of heat energy, one of the few methods for direct absorbed dose measurement
All energy absorbed in a material by radiation appears as as heat, used to calibrate ion chambers
Calorimetry/liquid dosimeter
4 advantages of calorimetry
Most basic method to measure absorbed dose compared to other dosimeters
Any conductive material with well-known thermal properties can be used as a sensitive volume
Absorbed dose is independent of energy
Stable against radiation damage at high doses
4 disadvantages of calorimetry
Insensitive: large dose for small temperature rise
Apparatus is large, bulky, difficult to setup, and move
Low spatial resolution
Slow to operate and takes a long time to reach equilibrium with surroundings
Dosimeter is a solution of ferrous sulfate (FeSO4)
When irradiated Fe2+ => Fe3+ charges oxidation, lose electron
Use spectrophotometry to measure Fe3+ concentration
Absorption peaks in UV light at specific wavelength
Ferric sulfate yields a G factor which is moles produced per joule absorbed energy, not very sensitive
Not common in RT clinics
Fricke (ferrous sulfate) chemical dosimeter
Device which releases light when heated following exposure to ionizing radiation
Materials are crystals with impurities that are “sensitive” to radiation
When irradiated, a small fraction of the absorbed dose is stored in the crystal lattice structure and when heated, some the stored energy is released in the form of light proportional to dose
In vivo dose measurement
Most commonly use lithium fluoride (LiF) which as Z of 8.2 which is close to soft tissue (7.4)
Thermoluminescent dosimeters (TLDs)
5 advantages of TLDs
Small size Reusability Wide dose range (0.001-1000 cGy) Near tissue equivalent = accurate reading based on Z No wire attachment
2 disadvantages of TLDs
Reading not instantaneous
Possible reading loss: only read once
Used for in vivo patient monitoring, in-house beam calibration, electron measurements, output constancy checks (morning warmup/daily QA)
Made of silicon crystal mixed with impurities to make a P-N type silicon
Silicon diodes
4 advantages of silicon diodes
High sensitivity
Instantaneous readout (in vivo)
Small size
Ruggedness/durable
4 disadvantages of silicon diodes
Energy dependence with photon beams
Directional/angular dependence must be considered if angle of beam isn’t perpendicular
Thermal effects: show a small temperature dependence
Radiation induced damage: high doses of radiation can displace silicon atoms from their lattice positions
Transparent film base coated with emulsion containing small silver bromide crystals
Ionizing radiation makes a chemical change within exposed crystals to form a latent image
Then developed and affected crystals get reduced to small grains of metallic silver and is then fixed and unaffected granules are removed by fixing solution resulting in a clear film in their place
Metallic silver is not affected by fixer and causes darkening
Degree of blackening is measured by determining optical density with a densitometer and is related to dose
Radiographic film
Light source, smaller aperture through which light is directed and a light detector to measure light intensity transmitted through film
Densitometer
Measure of light attenuated by film
Log(Io/Ix); Io = amount of light collected without film, Ix = amount of light transmitted through film
Optical density (OD)
Percent of transmitted light
It/Io x 100%
Net OD
Net OD = total OD - (base + fog)
Consists of ultrathin (7-23 um), colorless, radiosensitive leucodye bonded to 100 um thick mylar base or thin layer of radiosensitive dye sandwiched between two pieces of polyester base
Irradiation changes the colorless film to shades of blue as a result of polymerization; color stabilization takes about 24 hours, but most of the change occurs sooner
Radiochromic film
6 advantages of radiochromic film
Tissue equivalence: Z of 6-6.5
High spatial resolution
Large dynamic range of 0.01 Gy-1,000,000 Gy
Low energy dependence but can be somewhat sensitive to UV light and temperature
Insensitive to visible light and can be handled, stored, and worked with in normal room lighting
No chemical processing because it’s developed via radiation (up to 24 hours)
Ability to image two objects very close together
Spatial resolution
Material designed to mimic patient scattering and absorption for radiation measurement that can’t be tested on patient
Can put measurement device in this
Phantom
2 types of phantoms
Geometric
Anthropomorphic
Phantom in a simple geometrical shape that doesn’t imitate patient shape like cubes, rectangular slabs, cylinders, etc.
Ex: virtual water, water tanks for annual calibration
Geometric
Slabs of epoxy resin in solid form; geometric phantom
Virtual water
Phantom designed to mimic shape of average patient
Anthropomorphic
Material has the same or very similar radiation properties (scattering and absorption) as tissue with respect to a defined type of radiation
Water is the best substitute; Z of 7.5, 7 for tissue
Tissue equivalent
Sets US national standards for radiation quantities and measurements
National Standard Laboratories (NIST)
Standard temperature
22 C
_________ temperature, particles condense = lets more air into the chamber
Decreased
_________ pressure lets more air into the chamber; more air into the chamber = ________ ionizations reading
Increased; higher
Heating TLDs to release residual signs (previous electrons trapped from prior use) as well as condition sensitivity (erase)
Removes peaks one and two of the glow curve
Heat for 1 hour at 400 C then 24 hours at 80 C
Annealing
Plot of thermoluminescence against temperature
Glow curve
3 advantages of film
High spatial resolution
Permanent record
Inexpensive
4 disadvantages of film
Requires developing
Strong photon energy dependence: 10-50 times more sensitive at low energy photons (Z^3/E^3)
Not tissue equivalent
Sensitive to light
Radiosensitive transistor measures threshold voltage shift which is proportional to radiation dose
Standard sensitivity calibration factor = 1 mv/cGy; high = 9 mv/cGy (do well measuring low dose)
Connected to reader which measures threshold voltage shift (mV)
Sold withouthout buildup
Applied to patient’s skin and contained in the dark area at the end of the electrical lead
Metal oxide semiconductor-field effect transistors (MOSFETs)
3 advantages and 1 disadvantage of MOSFETs
Small and lightweight
Immediate dose reading
Reuseable
Limited life of 20,000 mV = 20,000 cGy with standard sensitivity
Very soft, low energy
20 kV
Low penetration depth limits applications, no longer used
Treatment for skin disease, mycosis fungoides, dermatitis, psoriasis, eczema
Absorbed in first 2 mm of skin
Grenz-ray therapy
40-50 kV; tube current of 2 mA
Good for tumors 1-2 mm deep, beam absorbed completely within 2 cm of tissue; can treat superficial rectal cancers
Contact therapy
Endocavitary machine
50-150 kV; tube current of 5-8 mA
Treat tumors 5 mm deep
Superficial therapy
150-500 kV, 10-20mA
Max dose is close to skin surface, 90% of the max occurs at 2 cm
Biggest limitation is skin dose, so we treat tumors until skin tolerance is reached
Orthovoltage/deep therapy
500-1,000 kV
Major problem was insulating the high voltage transformer which led to resonant transformer
Supervoltage/high-voltage therapy
1MV or higher
Megavoltage therapy
5 examples of megavoltage therapy
Van de Graff generator Linear accelerator (linac) Betatron Microtron γ-ray units like Co-60
2 MV up to 25 MV
No longer used due to better technology available, impractical due to size
Van de Graff generator
Accelerates electrons along the length of the waveguide to almost the speed of light
High frequency waves accelerate charged particles which can be used to treat, or can strike a target to produce x-rays
Have almost completely replaced every other type of EBT
Produces x-rays and heat via Brems
Energies range from 4 MV-22 MeV; most have dual-energies of 6 MV and 15 MV or 6 MV and 18 MV
Linear accelerator (linac)
Therapy at distance, RT delivered with an external beam
Teletherapy
2 major classes of teletherapy treatment units
Accelerators
Radioactive isotopes
Employ electric fields to accelerate charged particles
Teletherapy accelerators
2 types of teletherapy accelerators
Linear
Circular
Cobalt-60 (Co-60) has relatively low penetrating power
Average energy = 1.25 MeV; Dmax = 0.5 cm
Don’t use for abdomen because short falloff area; doesn’t carry energy through water well so would really have to irradiate skin surface to get dose through versus 18X doesn’t treat skin as well because of slow buildup
Radioactive isotopes
3 major linac vendors
Varian
Elekta
Siemens
Handed off forward and reverse waves combine to make a stationary wave
Standing waves
Riding a wave that terminates in the end
Traveling waves
Attenuates high energy photons from the forward peaked photon beam
Sits on carrousel and made of lead, uranium, aluminum, tungsten
Makes the dose profile of the beam uniform and reduces the overall dose rate by about four times, cuts dose rate in quarters (attenuates a lot of beam)
Specific to beam energy, different energies have different forward peaks
Takes a forwarded beak beam and flattens it out for an even dose profile at 10 cm depth
At shallow depths (less than 10 cm) beam intensity higher (overcompensates) at edges and creates lateral horns; after 10 cm, gets more intense in middle/forward peaker
Has higher average energy
Flattening filter
Flatness and symmetry percent
Flatness: 3%
Symmetry: 2%
If you take two points equidistant from central axis (CA), beam has to be the same by 2%
Symmetrical
80% of field size must be within 3% (dosimetrically middle of beam)
All doses taken within this area must be within 3% of each other, penumbra on edges
Variation of dose in comparison to the CA dose (80% of field) at a depth of 10 cm
Flatness
Beam has more intensity in the center without a flattening filter; higher dose rates can be given
Faster treatment because intensity not being reduced
Stereotactic (SRS) = high dose, fewer fractions (5 or less treatments)
Flattening filter free (FFF)
FFF 2400 cGy/min to _____ cGy/min with flattening filter
600 cGy/min
Point source where electrons naturally diverge are a thin beam
Virtual source
1 cGy at a standard depth (usually Dmax) for a 10x10 cm^2 field size at 100 cm away
RT unit of time
Monitor unit (MU)
4 sections of linacs
Patient support assembly (PSA)/couch
Gantry
Stand: motor, water storage, power components
Modulator cabinet
Rotates around isocenter of patient
Gantry
Power distribution center
Modulator cabinet
First used in 1950s
6-40 MeV
Has electron and photon mode
Accelerating tube is a hollow “doughnut,” placed between poles of an alternating current magnet
Pulsed electrons injected in the doughnut, an increasing magnetic field accelerates electrons which are made to strike a target
Downside = low photon dose rate due to the high need for electrons; about 1,000 times the need for photons compared to electrons alone
Accelerates electrons in circle
Betatron
Oscillating electric field of a microwave accelerates electrons
Magnetic field moves electrons in a circular orbit
More energy of electrons = increased radius of travel
A deflection tube extracts electrons
Microtron
Like a CT machine, the linac rotates around the patient and delivers radiation
Has CT imaging capabilities
Couch moves while beam is on (helical treatment)
No issues with field matching or abutting fields
6 MV for treatment, 3.5 MV for imaging; no flattening filter or field size limitations
Tomotherapy
Particle accelerator/reactor
Machine is very large and expensive (multi-story)
Particles are accelerated in a circular motion to increase energy, then a target or can be used to treat with particles
One can support 3-4 gantry heads
Main clinical use is for proton treatment; hydrogen gas is main source of protons
Accelerating heavy hydrogen or deuterons, also can be used to make radionuclides
Cyclotron
Increase energy = _______ depth of Bragg Peak
Increased
Depth dose curve shows the Bragg Peak, which has a steep drop off after Dmax
Typical energy = 70-250 MV
Multiple Bragg Peaks (Spread out Bragg Peak) are made via a variety of energies which are used to cover the entire target/treat tumors at certain depths
Modulator wheel: a circular inclined plane; various filtration limits energies to give an energy range
Proton therapy
Produced by irradiation Co-59 with neutrons in a reactor
Source inside in a stainless steel capsule, then placed in another steel capsule to prevent leakage
Undergoes beta decay and emits 1.17 & 1.33 MeV and emitted γ-rays are what treats the patient
Radionuclide machines
Cobalt 60
Co-60 penumbra is ______ due to the source size (1-2cm)
Large
Dose transition region at edge of radiation beam, over which dose rate changes rapidly as a function of distance from beam axis
Dose rate rapidly decreases near the lateral borders of the beam
Penumbra
Geometric penumbra
P = S(SSD + d - SDD)/SDD
P = penumbra S = Source size SSD = Source to skin distance SDD = Source to diaphragm (collimator) distance d = depth
Spread of dose near field borders, specified by lateral width of isodose levels (90-20% lines)
Physical penumbra
3 things physical penumbra depends on
Geometric penumbra
Depth
Beam energy
Electrons have a ______ penumbra, as they are negatively charged, and repel each other
Therefore larger block margins are needed to cover a target because penumbra is blurred
Supplemental lead shielding sharpens penumbra
Larger
Increase energy = _______ skin dose but ______ dose at depth
Decrease; increase
A 6X beam loses ____% of its energy per cm; 50% of beam left at _____ cm (more skin dose)
3%; 16.6 cm
A 18X beam loses ____% of its energy per cm; 50% of beam left/halfway attenuated at _____ cm (deeper)
2%; 25 cm
First ___-___ mm of patient usually skin; let treatment planning computer know
4-5 mm
Increase source size, depth, and SSD = _______ penumbra; increase SDD = _______ penumbra
Increase, decrease
