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RAD 380 - Physics of Radiation Therapy > Test 3 > Flashcards

Test 3 Flashcards

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1
Q

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

A

Ionization

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2
Q

Occurs when a charged particle such an electron, proton, or alpha particle collides with matter to produce a charged particle; interacts with tissue

A

Directly ionizing

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3
Q

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

A

Indirectly ionizing

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4
Q

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

A

Fluence

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5
Q

Fluence per unit time

A

Fluence rate/flux density

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6
Q

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

A

Exponential; hardened

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7
Q

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

A

Beam hardening

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8
Q

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

A

Good geometry

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9
Q

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

A 
Coherent scattering
Photoelectric effect
Compton effect
Pair production
Photodisintegration
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10
Q

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

A

Photon energy; atomic number (Z)

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11
Q

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

A

Coherent scatter

Classical or Rayleigh scattering

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12
Q

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)

A

Photoelectric effect

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13
Q

Photoelectric effect Z dependance

A

(𝑍^3/𝐸^3 )

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14
Q

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

A

Compton effect

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15
Q

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)

A

Pair production

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16
Q

Energy given to each positron and electron during pair production

A

(hν - 1.02 MeV)/2

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17
Q

Kinetic energy loss per unit path (length)

A

Electron stopping power (MeV/cm)

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18
Q

Max range of electrons

A

10 cm

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19
Q

Electrons lose ____ MeV per cm

A

2 MeV

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20
Q

Neutrons interact by 2 processes

A

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

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21
Q

Energy range of neutrons

A

Above 10 MV

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22
Q

Represents dose versus depth

A

Dose-depth curve

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23
Q

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

A

Build-up region

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24
Q

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

A

Dmax

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25
Q

Region beyond where dose falls steeply and almost linearly; photons drops exponentially

A

Fall-off region

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26
Q

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

A

Photonuclear reactions (y,n)

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27
Q

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

A

Total mass attenuation coefficient (u/p)

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28
Q

2 interactions between particles

A

Elastic collision

Inelastic collision

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29
Q

Total kinetic energy of all particles is the same before and after collision

A

Elastic collision

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30
Q

Some energy lost and goes to excitation, ionization, or brems

A

Inelastic collision

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31
Q

2 means by which electrons lose energy

A

Collisional

Radiative

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32
Q

Electron interacts with another electron

A

Collisional

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33
Q

Electron interacts with a positively charged nucleus

A

Radiative

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34
Q

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

A

Electron contamination

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35
Q

Distance a charged particles travels before coming to rest

A

Range

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36
Q

Heavy charged particles must have superimposed polyenergetic beams for sufficient tumor coverage

A

Spread-out Bragg peak

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37
Q

Heavy charged particles have a sharp peak in energy deposition near the end of the track

A

Bragg peak

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38
Q

TD5/5 of skin

A

5000 cGy per 100 cm^2

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39
Q

Amount of x or y-radiation to produce reddening of skin

A

Skin erythema dose (SED)

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40
Q

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

A

Exposure (X)

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41
Q

Traditional and SI unit of exposure

A

Traditional: Roentgen (R)
SI: Coulomb/kg of air

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42
Q

Unit of charge

A

Coulomb

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43
Q

Establishes standards for radiation, quantify volume of tumors and how they’re treated

A

International Commission on Radiation Units and Measurements (ICRU)

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44
Q

Total number of particles entering a sphere of small cross-sectional area; flow rate of photons
Units per area (m^-1 or cm^-1)

A

Fluence

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45
Q

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

A

KERMA

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46
Q

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

A

Absorbed dose

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47
Q

X-ray photon interacts with an outer shell electron with sufficient energy to eject it from orbit and alter its own path

A

Scatter

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48
Q

2 factors that cause surface dose

A

Backscatter

Electron contamination

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49
Q

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

A

Backscatter

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50
Q

Dose delivered at center of a sphere of a medium which is just large enough to have electronic equilibrium at its center

A

Dose in free space (Dfs)

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51
Q

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

A

Monte Carlo Algorithm

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52
Q

Average energy deposited per unit path length to a medium by ionizing radiation as it passes through that medium; deciding factor for quality factor

A

Linear energy transfer (LET)

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53
Q

4 main applications of radiation measurement instruments

A

Radiation machine calibration
Survey work
Personnel monitoring
In vivo patient measurements

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54
Q

Mean energy to produce ion pair

A

33.97 eV/ion pair

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55
Q

Exposure to dose conversion ratio that depends on medium and beam energy (will drastically change for low energy)

A

Fmed

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56
Q

Exposure formula

A

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
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57
Q

Chamber exposure calibration factor given by calibration lab; how much output electrometer has

A

Nx

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58
Q

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

A

Correction for ion recombination (Cion)

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59
Q

Standard pressure

A

760 mmHg

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60
Q

Ctp formula

A

Ctp = (760/P)(273+C/295)

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61
Q

Boiling points of fahrenheit and celsius

A

212 F

100 C

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62
Q

Fahrenheit to celsius formula

A

C = (F-32)(5/9)

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63
Q

Celsius to fahrenheit formula

A

F = (C x 9/5) + 32

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64
Q

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

A

Dose in free space (Dfs)

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65
Q

Dose formula

A

D = Fmed (X) (Aeq)

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66
Q

Very accurate measurement of radiation amount emitted by radiation producing device, not sensitive as radiation level is already very high

A

Radiation machine calibration instruments

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67
Q

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

A

Survey work

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68
Q

Track radiation worker doses
Need to be sensitive and measure small amounts of radiation
Must be able to measure cumulative radiation exposure

A

Personnel monitoring

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69
Q

Monitor amount of radiation patients receive during treatment

A

In vivo patient measurements

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70
Q

3 categories of radiation detectors classified according to medium used for detection

A

Gas ionization detectors
Solid-state detectors
Liquid dosimeters

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71
Q

3 gas ionization detectors

A

Ion chambers
Proportional counters
Gieger-Muller (GM) counters

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72
Q

6 solid-state detectors

A 
Thermoluminescent dosimeters (TLDs)
Film
Diodes
Metal oxide semiconductor-field effect transistors (MOSFETs)
Polymer gel
Scintillation
73
Q

2 liquid dosimeters

A

Calorimeters

Chemical

74
Q

Collect charge; very accurate measurement of amount of radiation
Beam calibration and survey meters

A

Ionization chamber

75
Q

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

A

Gas ionization chamber/detector

76
Q

Amount of ionization in air is dependent upon photon energy; increase energy = _______ ionization rate

A

Increase

77
Q

Ion chambers collect current, measures charge

A

Electrometer

78
Q

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

A

Electronic equilibrium

Dmax

79
Q

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)

A

Free-air ion chamber

80
Q

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

A

More

81
Q

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

A

Cavity ionization chamber

82
Q

3 majors types of cavity ion chambers

A

Thimble
Flat-cavity/plane parallel
Well-ionization for brachytherapy

83
Q

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

A

Thimble chamber

84
Q

3 ideal chamber characteristics

A

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

85
Q

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

A

Farmer chamber

86
Q

3 electrodes in well-guarded ion chamber of farmer chamber

A

Central/collector electrode
Guard electrode
Thimble wall

87
Q

Collects ionization charge and delivers current to the charge measuring device; 1 mm aluminum rod

A

Central/collector electrode

88
Q

2 purposes of guard electrode

A

Prevent leakage current from collector electrode

Define ion collecting volume

89
Q

Ground potential and kept at same potential as collector electrode (300 V)

A

Thimble wall

90
Q

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

A

Stem effect/leakage

Correction for stem leakage (Cst)

91
Q

Increase energy = _________ stem effect

A

Increased

92
Q

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

A

Parallel plane chamber/pancake

93
Q

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

A

Survey meter ion chamber

94
Q

1 Roentgen (R) = ______ rad in air

A

0.876 rad in air

95
Q

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

A

Gieger-Muller (GM) counters

96
Q

1 rad = ____ cGy = ____ ergs/g

A

1 cGy = 1 ergs/g

97
Q

1 rad = ____ J/kg = ____ Gy

A

1x10^-2 J/kg = 0.01 Gy

98
Q

Ratio of energy fluence at center of equilibrium mass to the same point in free air space
Increases with decrease in energy

A

Transmission factor (Aeq)

99
Q

6 radiation dosimeters

A 
Calorimetry
Chemical dosimetry
Solid state methods
Silicon diodes
Radiographic film
Radiochromic film
100
Q

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

A

Calorimetry/liquid dosimeter

101
Q

4 advantages of calorimetry

A

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

102
Q

4 disadvantages of calorimetry

A

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

103
Q

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

A

Fricke (ferrous sulfate) chemical dosimeter

104
Q

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)

A

Thermoluminescent dosimeters (TLDs)

105
Q

5 advantages of TLDs

A 
Small size
Reusability
Wide dose range (0.001-1000 cGy)
Near tissue equivalent = accurate reading based on Z
No wire attachment
106
Q

2 disadvantages of TLDs

A

Reading not instantaneous

Possible reading loss: only read once

107
Q

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

A

Silicon diodes

108
Q

4 advantages of silicon diodes

A

High sensitivity
Instantaneous readout (in vivo)
Small size
Ruggedness/durable

109
Q

4 disadvantages of silicon diodes

A

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

110
Q

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

A

Radiographic film

111
Q

Light source, smaller aperture through which light is directed and a light detector to measure light intensity transmitted through film

A

Densitometer

112
Q

Measure of light attenuated by film

Log(Io/Ix); Io = amount of light collected without film, Ix = amount of light transmitted through film

A

Optical density (OD)

113
Q

Percent of transmitted light

A

It/Io x 100%

114
Q

Net OD

A

Net OD = total OD - (base + fog)

115
Q

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

A

Radiochromic film

116
Q

6 advantages of radiochromic film

A

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)

117
Q

Ability to image two objects very close together

A

Spatial resolution

118
Q

Material designed to mimic patient scattering and absorption for radiation measurement that can’t be tested on patient
Can put measurement device in this

A

Phantom

119
Q

2 types of phantoms

A

Geometric

Anthropomorphic

120
Q

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

A

Geometric

121
Q

Slabs of epoxy resin in solid form; geometric phantom

A

Virtual water

122
Q

Phantom designed to mimic shape of average patient

A

Anthropomorphic

123
Q

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

A

Tissue equivalent

124
Q

Sets US national standards for radiation quantities and measurements

A

National Standard Laboratories (NIST)

125
Q

Standard temperature

A

22 C

126
Q

_________ temperature, particles condense = lets more air into the chamber

A

Decreased

127
Q

_________ pressure lets more air into the chamber; more air into the chamber = ________ ionizations reading

A

Increased; higher

128
Q

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

A

Annealing

129
Q

Plot of thermoluminescence against temperature

A

Glow curve

130
Q

3 advantages of film

A

High spatial resolution
Permanent record
Inexpensive

131
Q

4 disadvantages of film

A

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

132
Q

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

A

Metal oxide semiconductor-field effect transistors (MOSFETs)

133
Q

3 advantages and 1 disadvantage of MOSFETs

A

Small and lightweight
Immediate dose reading
Reuseable

Limited life of 20,000 mV = 20,000 cGy with standard sensitivity

134
Q

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

A

Grenz-ray therapy

135
Q

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

A

Contact therapy

Endocavitary machine

136
Q

50-150 kV; tube current of 5-8 mA

Treat tumors 5 mm deep

A

Superficial therapy

137
Q

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

A

Orthovoltage/deep therapy

138
Q

500-1,000 kV

Major problem was insulating the high voltage transformer which led to resonant transformer

A

Supervoltage/high-voltage therapy

139
Q

1MV or higher

A

Megavoltage therapy

140
Q

5 examples of megavoltage therapy

A 
Van de Graff generator
Linear accelerator (linac)
Betatron
Microtron
γ-ray units like Co-60
141
Q

2 MV up to 25 MV

No longer used due to better technology available, impractical due to size

A

Van de Graff generator

142
Q

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

A

Linear accelerator (linac)

143
Q

Therapy at distance, RT delivered with an external beam

A

Teletherapy

144
Q

2 major classes of teletherapy treatment units

A

Accelerators

Radioactive isotopes

145
Q

Employ electric fields to accelerate charged particles

A

Teletherapy accelerators

146
Q

2 types of teletherapy accelerators

A

Linear

Circular

147
Q

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

A

Radioactive isotopes

148
Q

3 major linac vendors

A

Varian
Elekta
Siemens

149
Q

Handed off forward and reverse waves combine to make a stationary wave

A

Standing waves

150
Q

Riding a wave that terminates in the end

A

Traveling waves

151
Q

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

A

Flattening filter

152
Q

Flatness and symmetry percent

A

Flatness: 3%
Symmetry: 2%

153
Q

If you take two points equidistant from central axis (CA), beam has to be the same by 2%

A

Symmetrical

154
Q

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

A

Flatness

155
Q

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)

A

Flattening filter free (FFF)

156
Q

FFF 2400 cGy/min to _____ cGy/min with flattening filter

A

600 cGy/min

157
Q

Point source where electrons naturally diverge are a thin beam

A

Virtual source

158
Q

1 cGy at a standard depth (usually Dmax) for a 10x10 cm^2 field size at 100 cm away
RT unit of time

A

Monitor unit (MU)

159
Q

4 sections of linacs

A

Patient support assembly (PSA)/couch
Gantry
Stand: motor, water storage, power components
Modulator cabinet

160
Q

Rotates around isocenter of patient

A

Gantry

161
Q

Power distribution center

A

Modulator cabinet

162
Q

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

A

Betatron

163
Q

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

A

Microtron

164
Q

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

A

Tomotherapy

165
Q

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

A

Cyclotron

166
Q

Increase energy = _______ depth of Bragg Peak

A

Increased

167
Q

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

A

Proton therapy

168
Q

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

A

Cobalt 60

169
Q

Co-60 penumbra is ______ due to the source size (1-2cm)

A

Large

170
Q

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

A

Penumbra

171
Q

Geometric penumbra

A

P = S(SSD + d - SDD)/SDD

P = penumbra
S = Source size
SSD = Source to skin distance
SDD = Source to diaphragm (collimator) distance
d = depth
172
Q

Spread of dose near field borders, specified by lateral width of isodose levels (90-20% lines)

A

Physical penumbra

173
Q

3 things physical penumbra depends on

A

Geometric penumbra
Depth
Beam energy

174
Q

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

A

Larger

175
Q

Increase energy = _______ skin dose but ______ dose at depth

A

Decrease; increase

176
Q

A 6X beam loses ____% of its energy per cm; 50% of beam left at _____ cm (more skin dose)

A

3%; 16.6 cm

177
Q

A 18X beam loses ____% of its energy per cm; 50% of beam left/halfway attenuated at _____ cm (deeper)

A

2%; 25 cm

178
Q

First ___-___ mm of patient usually skin; let treatment planning computer know

A

4-5 mm

179
Q

Increase source size, depth, and SSD = _______ penumbra; increase SDD = _______ penumbra

A

Increase, decrease

RAD 380 - Physics of Radiation Therapy (3 decks)

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