Test 3 Flashcards by Courtney W

Process of an atom acquiring a positive or negative charge; radiation strips an electron from a neutral atom to create a negative ion

Ionization

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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

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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

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Number of photons that pass through an imaginary cross section of a sphere; flow rate of beam, how much radiation is going through

Fluence

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Fluence per unit time

Fluence rate/flux density

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If a photon beam is monoenergetic, attenuation will occur in an __________ manner; when beams are polyenergetic, then the beam is _________

Exponential; hardened

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Low energy photons are filtered out and the beam therefore acquires a higher average energy than before

Beam hardening

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To measure a beam’s transmission through an absorber, the measurement must be done with scattered photons not measured

Good geometry

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5 types of interactions that cause attenuation of a photon beam by an absorbing material

Coherent scattering
Photoelectric effect
Compton effect
Pair production
Photodisintegration

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Each interaction has its own attenuation coefficient (μ/ρ) which varies with _________ and ____________

Photon energy; atomic number (Z)

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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

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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

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Photoelectric effect Z dependance

(𝑍^3/𝐸^3 )

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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

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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

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Energy given to each positron and electron during pair production

(hν - 1.02 MeV)/2

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Kinetic energy loss per unit path (length)

Electron stopping power (MeV/cm)

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Max range of electrons

10 cm

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Electrons lose ____ MeV per cm

2 MeV

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Neutrons interact by 2 processes

Recoiling protons from hydrogen and recoiling heavy nuclei from other elements
Nuclear disintegrations

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Energy range of neutrons

Above 10 MV

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Represents dose versus depth

Dose-depth curve

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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

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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

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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

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

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