ROQs Flashcards
(194 cards)
1
Q
What is bright on T1?
A
Fat
2
Q
What is bright on T2
A
CSF
3
Q
How does gray matter appear on T1?
A
darker than white matter
4
Q
How does gray matter appear on T2?
A
Brighter than white matter
5
Q
Ir-192 half-Life?
A
74 days
6
Q
Which unsealed source requires bystander shielding?
A
I-131, as it emits gamma rays.
Sources that release heavier particles (alpha, beta) don’t require shielding as these particles cannot penetrate enough.
7
Q
Pd-103 half-life?
A
17 days
8
Q
What does isobaric mean?
A
Same atomic mass (protons + neutrons): isobArs. They have different number of protons.
9
Q
What’re the products of B-plus decay?
A
Positron + neutrino
10
Q
What’re the products of B-minus decay?
A
electron and antineutrino
11
Q
What happens to Beta-plus (positron) particle after release?
A
It combines with electrons, annihilates, and releases two gamma rays (0.511 MeV each)
Used in PET imaging
12
Q
Why is there low dose distribution near the end of a brachytherapy source?
A
Anisotropy function:
The anisotropy means the dose is directionally dependent. Think of a radioactive seed as a paper towel roll tube. Holding the tube vertical and looking at the center of the tube, you see the whole tube. Now, rotate the tube to look through the hole like a telescope (come on, we’ve all done that). As the tube is rotated, you see less and less of it until all you see is the hole. For the brachytherapy source, as the measurement point moves from being directly perpendicular to the source, and “seeing” the entire source, to then moving to only “seeing” the tip end-on, the dose rate is going to be different (assuming the same distance from the source, etc.). The dose rate is different because of the self-shielding of the source (“seeing” less of the source), as well as variations in the cladding of the source, which tends to be thicker at the ends, depending on the manufacturer, which attenuates the gammas differently.
13
Q
What is the purpose of a beam spoiler?
A
It can increase surface dose through the generation of electrons by the spoiler.
A photon beam spoiler is a panel of low-Z material (often lucite) placed in the beam path close to the skin surface. Because photon interactions with the spoiler produce electron contamination, it is an alternative means to increase the surface dose. Spoilers do not meaningfully diminish beam penetration, unlike bolus, which diminishes beam penetration to a small degree.
Clinical pearl: Varying the thickness of the spoiler and its distance from the patient can be used to achieve or modulate the increase in superficial dose delivered by the treatment.
14
Q
How should the parallel opposed beams be weighted?
A
The beam closer to the target should be weighted more heavily. This minimizes hotspots w/o minimizing coverage.
15
Q
How does Strontium-89 decay?
A
Sr-89 decays via beta- decay, forming Yt-89
16
Q
What is Strontium-89 used for?
A
it is a radiopharmaceutical used to treat osseous mets. It is preferentially absorbed in the bones and releases B- particles within the metabolically active bone met.
17
Q
What is coherent scattering?
A
An incoming photon interacts with an electron and changes direction w/o losing energy. It’s in the name, scatter: nothing is created. Photons are merely scattered.
18
Q
What is internal conversion?
A
A higher-energy nucleus transfers energy to an orbiting electron, ejecting it.
Its in the name: conversion of energy into kinetic energy for an electron.
19
Q
What is the photoelectric effect (note NOT internal photoelectric)?
A
An incoming photon ejects an inner shell electron. An outer shell electron fills this vacancy, releasing energy in the form of characteristic XR.
20
Q
What is internal photoelectric effect (stress on internal)
A
Characteristic XR generated by photoelectric effect is absorbed by outer shell electrons, ejecting them (auger electron).
21
Q
What are the generic quality management tools in order of importance?
A
- Forcing functions and constraints
- Automation and computerization
- Protocols, standards, and information
- Independent double-check systems and redundancies
- Rules and policies
- Education
22
Q
What is a forcing function?
A
A forcing function is an aspect of a design that prevents the user from taking an action without consciously considering information relevant to that action. It “forces” conscious attention upon something.
23
Q
Why is Radium-226 no longer used in brachytherapy?
A
- Large in size, difficult to achieve conformal distributions
- LONG half-lives (1600 years) making it difficult to dispose
- Average emitted energy is high (0.83 MV (max 2.45 MeV)) making it difficult to shield.
However, it can be used with the Patterson-Parker system. The system was designed to be used with Ra.
24
Q
What is the half-life of Ra-226?
A
1600 years
25
What isodose line defines field size?
50%
26
What entity is proportional to the maximum XR photon energy in an XR tube?
Voltage
27
In an XR tube, what happens to the average XR energy if the voltage is increased?
Increases
28
In an XR tube, what is the relationship between the max energy of an XR and the average energy of the XRs?
Ave = 1/3 * Max
29
What does the picket fence test?
Individual and relative MLC positions
30
Why is secondary shielding used with LINACs performing IMRT vs. 3D?
To protect against secondary sources of radiation.
1. Scatter: Radiation scattered out of the patient. This depends on the dose delivered at the isocenter. This is usually significantly lower in energy than the primary beam.
2. Leakage: Radiation that escapes from the treatment head. With IMRT, there is an increase in the MU, therefore there is more leakage. Leaked radiation has the same energy as the primary beam, which needs additional shielding.
31
What is the difference between SSD and SAD setups?
32
What is the preferred dosimetric technique for measuring beam profiles for very small field sizes in SRS?
Film! Lol.
33
What is the rule of thumb for how much radioactive material decays per day during its first half-life?
1% per day
34
For I-125 implants, when can a patient be released to the general public?
When the dose rate at 1 m is 0.01 mSv/hr or total activity is < 9 mCi.
Ensure dose < 5 mSv to any individual (spouse, etc).
35
For Pd-103 implants, when can a patient be released to the general public?
When the dose rate at 1 m is 0.03 mSv/hr
36
For Ir-192 implants, when can a patient be released to the general public?
When the dose rate at 1 m is 0.008 mSv/hr
37
For I-131 implants, when can a patient be released to the general public?
When the dose rate at 1 m is 0.07 mSv/hr
38
What do TLDs and OSLDs measure?
Cumulative radiation exposure
39
What stimulates TLDs vs. OSLDs
TLDs: Stimulated by heat
OSLDs: Stimulated by light
40
Which can be re-read, TLDs or OSLDs?
OSLDs
TLDs can only me re-read once
41
Which can be reused after reannealing, TLDs or OSLDs?
Both
42
How do we measure beam flatness?
Note Max Intensity (Max) and Min (Min) intensity over the inner 80% of the field at 10 cm depth.
Then, F = (Max-Min)/(Max+Min)*100
43
What does beam flatness depend on?
Field size, beam energy, and depth.
44
How is electron beam energy defined when using a LINAC in electron mode?
The energy of the electrons at the entrance surface (NOT when it exits the machine)
45
Are electrons produced by a LINAC mono or polyenergetic?
Monoenergetic (all have roughly the same energy)
46
How is an electron beam spread out in a LINAC?
Using a scattering foil (as opposed to a target)
47
What is the maximum amount of energy shared by a photon with an electron in a Compton interaction?
75%
Occurs when a photon is backscattered and retains about 25% of the energy.
48
What is Compton scatter?
It is the principal interaction of photons with matter. Photon interacts with free or valence (loosely bound) electrons and scatters in a different direction, imparting some energy to the electron.
It is directly proportional to the number of outer shell electrons and the density of the material. Only weakly proportional to photon energy.
49
Does the linear attenuation coefficient depend on the density of a material?
Yes
50
What is the mass attenuation coefficient?
It is the linear attenuation coefficient normalized to density.
mass atten coeff = (lin atten coeff)/density
51
How do the linear attenuation coeff of water, ice, and steam compare?
Steam < Ice < Water
Water is denser than ice!
52
What is tissue-air ratio (TAR)?
It is the dose along the central axis of a beam as compared to some reference (air, etc)
53
What does tissue-air ratio (TAR) depend on?
TAR:
- Increases with beam energy and field size (2/2 increased contribution from scatter photons)
- Decreases with increasing depth 2/2 attenuation
54
What does the percentage depth dose (PDD) depend on?
Increases with
- Beam energy 2/2 increased penetrance
- Increasing field size 2/2 increased scatter photons
- Increasing SDD
Decreased with
- Increasing depth 2/2 inverse square law and attenuation
55
Is PDD more convenient for SSD or SAD setups?
SSD
56
What is the advantage of pencil beam scanning vs. passive scattering when using protons?
PCB achieves more conformal proximal conformality. Distant conformality is the same between the two!
57
What is the disadvantage of pencil beam scanning for protons?
Can miss the tumor due to motion, therefore, it is necessary to gate (respiratory, etc)
58
What are the components of a passive scattering arrangement for protons?
- Scattering foil: Spreads out proton beam laterally
- Patient-Specific Apertures: Conform beam to the tumor shape
- Compensator: Shapes the range of the proton beam to conform to the distal end of the target. It spreads the Bragg peak, which results in a more proximal dose than with PCB.
59
What are the contributors to the primary dose along the central axis of a LINAC beam?
Primary beam energy, followed by beam scatter (secondary, usually <10%).
Note that the scatter component increased with depth (primary beam is attenuated and scattered), field size.
60
At what activity level is a sealed source considered leaky?
> 185 Bq
> 0.005 uCi
61
How does surface dose depend on beam energy and field size?
Increased by:
- increased field size
- deceased beam energy (up to 10 MV)
- Increased beam angle (beam enters obliquely)
- decreased tray-to-skin difference (dose from e-contamination)
62
What are the different wedges used?
- Physical (steel, lead): placed in the beam path, and produce the largest scatter
- Universal: located inside the LINAC head. Less scatter than a physical wedge.
- Dynamic (MLCs): Least amount of scatter. Uses variable dose rates
63
What is the half-life of Cs-137?
30 years
64
What is "quenching" in radiation detectors?
Prevents a single ionization event from registering as multiple events. So every event is "quenched?"
65
What is chemical "quenching" in radiation detectors?
Halogen gas additive in a radiation detector absorbs UV light emitted by single radiation events.
66
What is electronic "quenching" in radiation detectors?
Transient reduction in anode voltage after an event is detected. Leads to a dead time, during which another event cannot be recorded.
67
What is a half value layer?
It is the thickness of a material required to reduce radiation intensity to half its original value.
68
How to the first and second half value layers compare with one another (HVL1 vs. HVL2)?
For monoenergetic beams, HVL1 = HVL2.
For polyenergetic beams, the beam becomes hardened as it passes through the first HVL. Therefore, the second HVL is larger than the first (higher energy photons left): HVL2 > HVL1.
69
What peak kilovoltage (kVp) across an XR tube produces diagnostic XRs?
20-120 kVp
70
What peak kilovoltage(s) across an XR tube generate therapeutic XRs?
Superficial: 50-150 kV
Orthovoltage: 150-500 kV
Megavoltage: ≥ 1 MV
Note diagnostic XRs (20-120 kVP) and superficial XRs have a significant amount of overlap.
71
How do head scatter (Sc) and phantom scatter (Sp) vary with field size?
Increase with field size as more photons/electrons are available to be scattered into the central axis of the beam with increasing field sizes.
72
What part of the LINAC is responsible for the head scatter (Sc)?
The jaws are responsible for it. Tertiary field shaping (MLCs) also scatter, but this is negligible compared to the jaws.
73
What contributes to phantom scatter (Sp)?
Tertiary scatter (MLCs)
Field Size
74
What is the advantage of on-board MV-CBCT imaging as opposed to kV-CBCT?
Less susceptibility to artifacts caused by high Z objects (metallic implant/fillings).
75
What is the advantage of kV-CBCT over MV?
Better image quality at much lower doses.
76
What are isotones?
Same number of neutrons.
77
What are isotopes?
Same mass and atomic number but differ in their energetic states (one usually has a higher energy).
78
What are isotopes?
Same number of protons, but different number of neutrons.
79
What is the main drawback of using a DVH only to evaluate a plan?
Loss of spatial information, as we cannot tell where the hot or the cold spots are.
80
What is a stochastic process?
Stochastic = randomly determined
81
What causes a proton beam to deflect laterally when traveling through matter?
Multiple Coulomb Scattering (MCS): Repulsive/attractive forces between charged particles. In the case of proton beams and matter, these are the repulsive forces between the beam and matter protons.
82
What is range straggeling?
These are the variations in the distance protons will travel in matter, as this depends on individual proton interactions with matter. These can differ from proton to proton.
83
What MLC orientation compared to the axis of a lesion is best for the best dose modulation?
Perpendicular to the long axis of the lesion.
84
Which radiation(s) are directly ionizing?
Charges particles, like electrons and protons.
85
Which radiation(s) are indirectly ionizing?
Uncharged particles, such as photons and neutrons.
86
What is the dmax of a 6MV photon beam?
1.5 cm
87
What is the dmax of a 10MV photon beam?
2.5 cm
88
Dmax of a 15 MV photon beam?
3.0 cm
89
Dmax of a 18 MV photon beam?
3.5 cm
90
Dmax of a Co60 beam?
0.5 cm
91
PDD for a 6MV photon beam at 10 cm?
~67%
92
What is helical tomotherapy?
A treatment system (like a CT scanner), where the patient and the couch continuously move through a fan beam delivery system.
93
What is serial tomotherapy?
A treatment system (like a CT scanner), where the patient and the couch move step by step (sequentially through a fan beam delivery system.
94
What is fluence (Φ)?
Number of particles (N) incident on a sphere of cross-sectional area A:
Φ = N/A
95
What is energy fluence (Ψ
)?
Energy of all the particles (Et) incident on a sphere of cross-sectional area A:
Ψ = Et/A
96
What is kerma?
Kinetic
Energy
Released
per unit
MAss
Its units are J/kg or Gy
It is not a good measure of biologic damage, as it does not tell us how much energy is absorbed locally.
Total kerma is a sum of collisional and radiative kermas.
97
What is absorbed dose?
It is the total energy absorbed by a mass from ionizing radiation. Its units are J/kg or Gy
1 Gy = 100 rads (1 rad = 1cGy)
Absorbed dose is equal to collisional kerma.
98
What is a curie?
It is the activity of 1 g of Ra-226.
1 Ci is a large activity so it is mostly expressed in mCi.
99
What is the conversion between Curie (Ci) and becquerel (Bq)?
1 mCi = 37 mBq
100
What is becquerel (Bq)?
It is a unit of radioactivity
101
What is the order of LINAC components that electrons encounter?
1. Electron Gun: Releases e
2. Waveguide: Accelerates e to desired energies
3. Bending Magnet: Bends e towards the patient.
4. Target: Striking electrons release photons via bremsstrahlung.
5. Flattening filter: Preferentially attenuates center of the beam (higher energies) to create a uniform field of photons.
102
What is bremsstrahlung radiation?
It means "breaking radiation."
It is produced when charged particles (electrons) are decelerated by another charged particle (atomic nucleus, protons). The electrons lose kinetic energy, which is converted into radiation (photons).
103
How to the X-ray spectrums with no filtration, inherent filtration, and complete filtration compare?
The spikes represent characteristic XRs.
104
What is inherent filtration?
Some of the generated photons (lower energy) are absorbed by the target before they can exit the target. Only higher energy XRs, which have enough power to escape the target are released. This is called inherent filtration.
105
What is complete filtration?
This is when additional material is placed before a photon ray, which hardens the beam by preferentially absorbing lower energy photons.
106
How are adjacent MLCs designed?
They have a tongue-in-groove design, which prevents friction while also preventing radiation leakage between the leaves.
107
What is the approximate interleaf (between leaves) transmission, which is minimized by the tongue-in-groove design?
3%
108
What is the approximate transmission THROUGH the MLCs?
1-2%
109
What is the umbrella term for radiation delivery technique that required dynamic MLC movement?
Volumetric modulated ARC therapy: This includes VMAT, RapidArc, Tomotherapy, etc.
110
What is the half-life of I-125?
60 days
111
What is the total dose (TD) delivered by a permanent brachytherapy implant of half-life, µ, and initial dose rate?
First, calculate the mean life (ML) of an isotope
ML = 1.44 x µ
TD = ML x Initial Dose Rate
112
For LINACs, how often should you check the coincidence between kV imaging isocenter and MV treatment isocenter?
Daily!
Variations in this can directly affect patient setup integrity!
113
What is the recommended extent of CT sim cans above and below the target when treating with non-coplanar beams SBRT?
15 cm
114
What is the risk priority number (RPN)?
It indicates the risk posed to the patient by undetected failures. Occurrence (O), Severity (S), and Detectability (D) are combined into an RPN
RPN = O*S*D
Each metric is assigned a value between 1-10, thus RPN ranges between 1-1000. The higher the number, the higher the priority to address it in prospective risk analyses.
115
What defines the QA tolerances for linear accelerators?
QA tolerances for parameters such as localizing lasers, accuracy, imaging, and treatment coordinate coincidence, are based on the capability of the LINAC. QA tolerances become more stringent from non-IMRT < IMRT < SBRT/SRS.
116
For an x-bit grayscale image, how many shades of gray can it represent?
2^x
117
What is the conversion from bit to byte?
8 bit = 1 byte
118
What is the conversion from byte to kilobyle?
1 kilobyte = 1024 bytes
119
What is the conversion from kilobyte to megabyte?
1 megabyte = 1024 kilobytes
120
What is the "quality" of a photon beam?
It is the energy distribution of the photons within the beam.
121
What is the nominal accelerating potential (NAP)?
It is the maximum energy the photons in a beam contain. It is what we typically refer to when we say 6 MV, 10 MV, etc.`
122
What is the best measure of energy (beam quality) for kV beams?
HVL!
123
What is the best measure of energy (beam quality) for MV beams?
PDD!
124
What is the electromagnetic spectrum?
125
What is the key (definition) difference b/w X rays and gamma rays?
Gamma rays are produced within the nucleus (radioactive decay)
X-rays are produced extranuclear-ly (by abruptly decelerating high-energy electrons) → Bremsstrahlung
126
How does the LET of neutrons compare to photons?
They have a higher LET, resulting in more efficient cell killing (more radiobiological effect).
127
Do neutrons primarily interact with the nucleus or the orbital electrons?
Because of their higher mass, they mainly interact with the nucleus.
128
Which material is the best absorber of neutrons?
Hydrogen-rich materials (concrete, etc) are the best absorbers of neutrons. Hydrogen nuclei (protons) are similar in mass to neutrons, thus, a neutron can lose all of its energy when it collides with a proton.
129
What is photodisintegration?
The process by which a nucleus absorbs a high-energy photon and ejects a neutron (termed photoneutron). This happens above 10 MV!
130
What is the threshold for photoneutron generation?
10 MV
131
How is the photoelectric effect used in medical imaging?
132
What kind of photon-matter interaction predominates in diagnostic XR imaging?
Compton predominates above 25 keV (20-150 kVp). It depends on the density of the material, and is responsible for soft tissue contrast.
133
What does the photoelectric mass attenuation coefficient depend on?
It depends on the atomic number cubed (Z^3). Makes sense, since it is ejecting electrons.
134
What is the difference between the photoelectric effect and Compton scattering?
It's in the name!
Compton is photon scattering after a collision! It is a higher energy phenomenon.
While in the photoelectric effect, the photon is fully absorbed to release a photoelectron. It is a lower energy phenomenon.
135
What does the Nuclear Regulatory Commission (NRC) regulate?
Regulates all nuclear materials used both in medicine and the industry.
136
Which regulatory body regulates medical LINACs?
The FDA!
137
What is the meaning of anisotropy?
An - Not
isotropy - Same property in all directions
Thus, anisotropy means something that does not have the same property in all directions (it is directionally dependent).
It's most often used to apply to brachytherapy sources and dose calculations.
138
How are intensities (I) related to distances (r) from a radiation source?
(I1/I2) = (r2/r1)^2
It's the inverse square law!
139
Mnemonic for EM spectrum from highest to lowest frequency?
Good Xylophones Use Very Important Music Recorders
Gamma > XR > UV > Visible > IR > Micra > Radio
140
Why do we rotate MLCs during IMRT?
To reduce dose accumulation of interleaf leakage in one plane only.
141
What is the k-edge in XR spectroscopy?
k-edges are a sharp increase in attenuation coefficient of the photoelectric effect when the photon energy is just above the threshold binding energy of the absorbing k-shell electron.
142
What makes barium and iodine effective contrast agents?
They have properties conducive to photon absorption through the photoelectric effect.
High-Z (remember photoelectric attenuation coefficient is proportional to Z^3)
They have k-edges in the diagnostic XR range!
143
What is the equation of the XR output of an XR tube?
Output = tube current x exposure time x kVp^2
144
What is the typical dose uniformity goal or TBI?
+/- 5%
145
What is the function of primary barriers in a radiation vault?
To protect against primary radiation (radiation emitted directly from the source)
146
What is the function of secondary barriers in a radiation vault?
To protect against secondary sources of radiation:
1. Scatter
2. Leakage
147
What is the mean energy of I-125
30 keV (mnemonic: it is ~ half it's half life (60 days))
148
How does I-125 decay?
Decays via electron capture and subsequent gamma release.
149
What is the HVL of I-125 in lead?
0.025 mm
150
How does Pd-103 decay?
It decays via electron capture and emission of gamma rays, just like I-125.
151
What is the average radiation energy of Pd 103?
21 keV
152
What is the HVL of Pd-103 in lead?
0.0085 mm
153
What is the average radiation energy of Ir-192?
372 keV OR 0.38 MeV
Higher than other sources, which makes sense since it is used for HDR.
154
How does Ir-192 decay?
It decays via B minus and gamma emission.
155
How does Cs 137 decay?
It decays via B minus and gamma emission.
156
What is the average radiation energy of Cs-137?
662 kEV or 0.662 MeV
157
What is the HVL of Cs-137 in lead?
5.5 mm
158
What is the HVL or Ir-192 in lead?
2.5 mm
159
What radioisotopes are produced by neutron activation?
COPD eeriee
Co-60, Pd-103, Ir-192, I-125
160
Which radioisotopes are produced by nuclear fission?
Scissor
Cs-137, I-131, Sr-90
161
What is the maximum number of electrons that can fit into a shell?
2*n^2 (n is the shell number)
162
What is the range (in cm) of electrons in water and air?
Rule of thumb: In water, the range is incident energy (MeV) divided by 2.
In air, we have to apply a density correction factor:
Range = (E*(Water_den/Air_den))/2
163
What is the formula for various isodose lines for electrons of incident energy Eo?
5-4-3-2 rule! (Dmax, D90, D80, Range)
4 is actually 3.2-3.3
Dx = Eo/x
(X is either 5, 4, 3, or 2 depending on which depth you want!)
164
What is the physical quantity responsible for dmax of a MV photon beam?
It is the maximum range of the electrons set in motion by the MV photons.
165
What is the dmax of a 4 MV photon beam?
1 cm
166
Which governing body sets the standards for radiation exposure within the united states?
The National Council on Radiation Protection and Measurements (NCRP)
167
What is the average public radiation exposure for adults?
Daily: 0.017 mSv per day
Annual: 6.2 mSv per yr
168
What constitutes the average public radiation exposure?
This exposure is a sum of medical and background radiation exposure. Medical exposure makes up around 48% of the exposure.
169
What % of the average public radiation exposure for US adults is due to medical tech? What is the breakdown with respect to tech?
48%
CT - 24%
Nuc Med - 17%
Fluoroscopy - 7%
170
What % of the average public radiation exposure for US adults is due to background radiation? What is the breakdown with respect to sources of such radiation?
Radon - 37%
Cosmic - 5%
Consumer radiation exposure - 2%
171
What is the annual NCRP dose limit for occupation radiation exposure for adults?
50 mSv
172
What is the NRCP effective dose limit for the general public?
1 mSv/year
This is the limit for people exposed to radiation from man-made sources, but it does NOT include exposure from medically necessary imaging and treatment (eg RT) procedures. It also does NOT include exposure to background radiation.
173
What is the NCRP radiation dose limit for the fetus of a pregnant radiation worker?
5 mSv/year
1/10 the limit to a non-pregnant worker
174
Why do heavy ions have straight ionization tracks, while electrons have scattered tracks?
Because electrons have significantly less mass than heavy ions! This is particularly true for heavy Z materials!
175
Dose calibrations for MV photon and electron beams must be performed in what material?
In water (30 x 30 x 30 cm tank) ONLY
176
What are beamlets?
Beamlets are a division of the beam into an array of small beams from the beams eye view, which allows for the generation of fluence and intensity maps to assist in IMRT planning.
Each beamlet in 2D measures about 0.5 x 0.5 cm, or 1 x 1 cm.
177
What is the rest mass of an electron or a positron?
0.511 MeV
178
What is the minimum amount of photon energy required for pair production?
0.511 MeV x 2 = 1.022 MeV
This energy is used solely to create the mass of electrons and positrons. The remained energy (if a photon has higher energy) is imparted as the kinetic energy of the pair.
179
What is the density of water?
1 g/cm3
180
What is the formula for the effective depth of a beam as it passes through inhomogeneous tissues?
Dep_eff = (dep_1*den_1) + (dep_2*den_2)...
181
What is the total mass absorption coefficient?
It is the sum of the mass absorption coefficients for the photoelectric effect, Compton effect, and pair production.
Its coefficients are cm^2/gm. It is independent of mass, unlike the density absorption coefficient.
It is DEPENDENT on energy.
182
What attenuation per mm of lead (shielding/blocking) completely blocks an electron beam relative to beam energy?
2 MeV electron energy is completely blocked by every mm of lead.
Therefore, beam energy/2 (+1mm) is the length of the lead block required. 1 mm is added for safety.
183
Why are cerrobend blocks used instead of lead in clinical practice?
Cerrobend is a lead allows. It is used because of its lower melting point, and therefore ease of being shaped into custom blocks.
It is ~83% as dense as lead. So for every mm of lead, 1.2 mm of Cerrobend is required to achieve the same shielding effect.
184
How many mm of Cerrobend relative to lead are required to achieve the same shielding?
1.2 mm of Cerrobend per mm of lead
Cerrobend is only ~83% as dense as lead.
185
What is the hinge angle?
It is the acute angle between the central axes of two beams.
186
What is the formula for calculating the optimal wedge angle?
Wedge angle = 90 - (hinge angle/2)
187
What radiation device can measure absolute dose?
"Freak" dosimeters
Now usually replaced by ion chambers.
188
Why is there a tail beyond the practical range of an electron dose distribution>
The tail is the result of electrons interacting with the high Z components of the treatment head (collimator, scattering foil, applicator). These generate bremsstrahlung XR contamination, which causes the PDD tail.
189
How does a LINAC achieve rapid dose fall-off for SRS/SBRT setups?
They use focus cones to minimize penumbra, which is critical since we are using very high energies. These cones are added to the collimator group, below the jaws.
Because this brings the tertiary collimator closer to the patient (increased length of the collimator), it increases the likelihood of collision with the patient!
190
What is the main mode of metabolism for cancer cells?
They rely on glycolysis much more than oxidative phosphorylation. This is the basis for FDG PET Scans,
191
What is the Winston-Lutz QA method? What does it test?
It is MV imaging to check the coincidence between the gantry isocenter and laser alignment.
192
What is the star shot QA test?
Determines radiation isocenter by exposing film to different collimator, gantry, and couch positions.
193
What is the Plastic cube with a metal ball in the center QA test?
It checks the coincidence between KV and MV imaging.
194
What is the end-to-end test and what does it test?
It carries out a treatment plan in a phantom (sim, plan, and treat the phantom). It needs to occur yearly.
