Key Facts + Eq Flashcards
1
Q
What is the formula for beam flatness?
A
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
2
Q
What is the formula for MU calculation using SSD setup for a square field?
A
MU = Dose/(K × PDD × Sc × Sp × OAF × TF)
- K = Output Factor
- PDD= Percentage depth dose (for eqsq on the phantom/patient (after MLC collimation))
- Sc = Head scatter correction (for eqsq for collimator setting (before MLC collimation))
- Sp = Phantom scatter correction (for eqsq on the phantom/patient (after MLC collimation))
- OAF = Off-axis factors
- TF= Transmission factors
3
Q
What’s the TMR for a 10x10 cm2 6 MV photon beam at 10 cm?
A
0.784
4
Q
What’s the PDD for a 10x10 cm2 6MV photon beam at 10 cm?
A
~68%
5
Q
What’s the average energy of Co-60?
A
1.25 MeV
6
Q
What’s the formula for inverse square factor?
A
ISF = [SSDref + dmax/SSD + dmax]2
7
Q
What are Sc and Sp (used in MU calculations) for a 10x10 field?
A
1
8
Q
What special steps do we need to take to calculate MU for a rectangular field?
A
Convert it to an equivalent square
Eq Sq = 4A/P
9
Q
What’s the equation relating the decay constant, γ, to t1/2?
A
t1/2 = ln2/γ
10
Q
What is the decay equation?
A
A(t) = Aoe-λt
Simplified to:
- A(t) = Aoe-0.693 x t/HL1/2
- A(t) = Ao x 1/2-t/HL
11
Q
What is the XR output of an XR tube?
A
Output = tube current x exposure time x (peak kVp)n
n is normally 2
12
Q
What is the conversion from Tesla (SI Unit) to Gauss (CGS units)?
A
Both measure magnetic flux (strength)
1T = 10,000 G
13
Q
What is the equivalent square of a circular field of radius r?
A
EqSq = r√ π
14
Q
How do you calculate TVL from μ or HVL?
A
15
Q
The formula for beam intensity after passing through x cm of material w/ a certain HVL:
A
- I = Io x (1/2)x/HVL
- I = Io x e-0.693x/HVL
- I = Io x e-μx
16
Q
The formula for HVL?
A
HVL = ln2/µ
ln2 = 0.693
17
Q
What is ln2?
A
0.693
18
Q
What is the conversion between Curie (Ci) and becquerel (Bq)?
A
1 Ci = 37 x 109 Bq
19
Q
What is fluence (Φ)?
A
Number of particles (N) incident on a sphere of cross-sectional area A:
Φ = N/A
20
Q
What is energy fluence (Ψ)?
A
Energy of all the particles (Et) incident on a sphere of cross-sectional area A:
Ψ = Et/A
21
Q
What’s the t1/2 of Co-60?
A
5.26 yrs
22
Q
Dmax of a Co 60 beam?
A
0.5 cm
23
Q
Dmax of a 18 MV photon beam?
A
3.5 cm
24
Q
Dmax of a 15 MV photon beam?
A
3.0 cm
25
What is the dmax of a 10MV photon beam?
2.5 cm
26
What is the dmax of a 6MV photon beam?
1.5 cm
27
What is the total dose (TD) delivered by a permanent brachytherapy implant of half-life, t1/2, and initial dose rate?
First, calculate the mean life (ML) of an isotope
- ML = 1/γ
- ML = 1.44 x t1/2
- TD = ML x Initial Dose Rate
28
For an x-bit grayscale image, how many shades of gray can it represent?
2x
29
What is the conversion from bit to byte?
8 bit = 1 byte
30
What is the conversion from byte to kilobyle?
1 kilobyte = 1024 bytes
31
What is the conversion from kilobyte to megabyte?
1 megabyte = 1024 kilobytes
32
How are intensities (I) related to distances (r) from a radiation source?
(I1/I2) = (r2/r1)^2
It's the inverse square law!
33
What is the equation of the XR output of an XR tube?
Output = tube current x exposure time x kVp^2
34
What is the mean energy of I-125?
35.5 keV
Mnemonic: it is ~ half it's half life (60 days)
35
How does I-125 decay?
Decays via electron capture and subsequent gamma release.
- e- caputure: 125I → 125Te
- Decay back to ground state by emitting 35.5 keV γ ray
36
What is the HVL of I-125 in lead?
0.025 mm
37
How does Pd-103 decay?
It decays via electron capture and emission of gamma rays, just like I-125.
38
What's the t1/2 of Pd-103?
17 days
39
What is the average radiation energy of Pd-103?
21 keV
40
What is the HVL of Pd-103 in lead?
0.0085 mm
41
What is the average energy and t1/2 of Ir-192?
- 72 days
- 0.38 MeV γ
Higher than other sources, which makes sense since it is used for HDR.
42
How does Ir-192 decay?
It decays via B minus and gamma emission.
43
How does Cs-137 decay?
It decays via B minus and gamma emission.
44
What's the t1/2 of Cs-137?
30 yrs
45
What's the t1/2 of Cs-131?
9.7 days
46
What's the average energy of Cs-131?
30 keV
47
How does Cs-131 decay?
e- capture and XR emission
48
What is the average radiation energy of Cs-137?
0.662 MeV
49
What is the HVL of Cs-137 in lead?
5.5 mm
50
What is the HVL or Ir-192 in lead?
2.5 mm
51
What is the maximum number of electrons that can fit into a shell?
2n2
n = shell number
52
What is the range (in cm) of electrons in water and air?
Rule of thumb: In water, 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
53
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 is actually 2.8
Dx = Eo/x
(X is either 5, 4, 3, or 2 depending on which depth you want!)
54
What is the dmax of a 4 MV photon beam?
1 cm
55
What is the average public radiation exposure for adults?
Daily: 0.017 mSv per day
Annual: 6.2 mSv per yr
56
How much of the average public radiation exposure for adults in the US is due to medical tech? What is the breakdown with respect to tech?
48%
CT - 24%
Nuc Med - 17%
Fluoroscopy - 7%
57
What percentage of the average public radiation exposure for adults in the US is due to background radiation? What is the breakdown with respect to sources of such radiation?
Radon - 37%
Cosmic - 5%
Consumer radiation exposure - 2%
58
What is the annual NCRP dose limit for occupation radiation exposure for adults?
50 mSv
59
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.
60
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
61
What is the rest mass of an electron or a positron?
0.511 MeV
62
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.
63
What is the density of water?
1 g/cm3
64
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)...
65
What is the formula for the amount of shielding required for a radiation bunker?
Shielding ∝ (workload x use factor x occupancy) / distance2
66
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.
- Approx 20% more cerrobend is required for blocking
67
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 lead block required.
68
What is the formula for calculating the optimal wedge angle?
Wedge angle = 90 - (hinge angle/2)
Hinge angle = gantry angle
69
How are the wedges arranged for opposed tangents for breast treatments?
The heels of the wedges face towards the anterior (nipple) part of the breast.
70
What is the thicker part of a wedge called?
The heel
71
What is the thinner part of the wedge called?
The toe
72
Under which part, the heel or the toe, is the dose distribution hot vs. cold?
Under the heel, the dose distribution is cold (more attenuation). Under the toe, the dose distribution is hot (less attenuation).
73
How often should the department QA committee meet?
At least monthly
74
How can you increase the maximum field length without increasing SSD?
Rotate the collimator by 45 deg.
The size would be √2* (f)
Where f is the field size.
75
Conversion from eV to J?
1.6 x 10^=10 J
76
Rest mass of the proton in eV?
938 MeV
77
The formula for effective half-life?
- 1/Teff = 1/Trad + 1/Tbio
- Teff < Trad or Tbiof
78
The formula for power?
Power = V x A
79
What is the Mayneord F-factor and what is the formula for calculating it?
The ratio of the inverse square component of PDD from the reference SSD to another SSD.
Because the dose does not fall off as rapidly at extended SSDs, the **F-factor is > 1** but only by **a few %**
Mnemonic: [(Old & deep (SSD1 + d) × new & shallow (SSD2 + dmax)) ÷ converse]2
80
What is the relationship between TVL and transmission?
- log (desired transmission/current transmission) * TVL
81
The dosimeter to measure fetal dose in a pregnant radiation worker is placed where? Does it measure daily, weekly, monthly, or yearly dose?
It's placed around the abdomen. It measure monthly dose (limit ≤ 50 mrem)
82
How should you orient the MLCs if you want to minimize head leakage?
90 (leaves travel parallel to the sup/inf axis of the patient
83
What does V mean in dose constraints?
V = volume receiving a dose ≥ X Gy
84
What does D mean in dose constraints?
The dose received by X % of the volume
85
What is the conversion between R and cGy?
1 R = 0.876 cGy
86
What is the conversion between Sv and rem?
1 Sv = 100 rem
87
The formula for MU calc for electron beams?
MU = Dose / (K x ISF x PDD x OF)
K = Output factor (usually 1 cGy/MU)
ISF = Inverse square factor using SSDeff
PDD = Prescription isodose line
OF = Obliquity factor (increase in dose with oblique beam entry)
88
What's the formula for PDD?
(Dose rate at depth / dose rate at dmax) x 100
89
The formula for the practical range (R_p) of electrons?
E/2
90
The formula for the range of 80% IDL (R_80) of electrons?
E/2.8
91
The formula for the 90%V IDL (R_90) of electrons?
E/3.3
92
What is the output factor (K) for an electron beam at d_max and 100 cm SSD?
1
93
What is the ISF for electron beams?
ISF = (SSDeff / (SSDeff + ΔSSD))^2
94
What's the formula for calculating CT numbers (Hounsfield units)?
CT number = 1000 x [{μ_mat - μ_water)/μ_water]
95
How does the range of e relate to electron density?
Higher the e-density, the shorter the range
e density is proportional to mass density (Hounsfield Units)
96
What's the beam energy for a tomotherapy unit?
6 MV only
97
What's the % attenuation for 6 MV and 15 MV photon beams?
- 6 MV: 3%/cm
- 15 MV: 2%/cm
98
What's the air kerma rate at the pubic symphysis for patients receiving I-125 seed implantation in the prostate?
25 μGy/h
99
What thickness of concrete is enough to shield Linacs outputting up to 18 MV of photons?
260 cm, or 8.6 ft
100
How much higher are the linac-leakage workloads for IMRT vs. conventional radiation?
2-10 x higher
101
What's the formula for the required barrier thickness?
- B = P × d2 / WUT
-- B = barrier transmission factor
-- P = permissible dose
-- d = distance from the source
-- W = **W**orkload, total **w**eekly radiation at 1 m from source
-- U = Use factor, fx of operating time during which a Linac is directed towards a particular barrier
-- T = occupancy factor; fx of operating time during which the area is occupied
102
What is the formula for permissible dose equivalent for an area?
W = workload at 1 m; total weekly radiation delivered
U = use factor; fx of operating time during which a Linac is directed towards a particular barrier
T = occupancy factor; fx of operating time during which the area is occupied
d = distance from the radiation source
B = transmission factor of a barrier
103
At which energies is the PE effect the most dominant interaction?
- dominant: 10 - 25 keV
- range: 1-150 keV
104
What does the probability of the PE effect depend on?
PE probability ∝ Z3/E3
105
At which energies is Compton scattering the most dominant interaction?
- dominant: 26 kEv - 24 MeV
- range: any
106
At which energies is pair production the most dominant interaction?
- dominant: > 10 MeV
- range: 1.02 MeV and above
107
What does the probability of the pair production depend on?
Probability is proportional to Z2 and increases dramatically w/ energy.
108
What's the neutron leakage for 18 MV photons?
0.15%
109
What's the neutron leakage for 10 MV photons?
0.04%
110
What's the t1/2 and average energy of Ra-226?
- 1601 yrs
- 0.83 MeV γ
111
What's the t1/2 and average energy of Rn-222?
- 2.7 days
- 0.83 MeV γ
112
How do you calculate the total uncertainty of a treatment delivery setup?
Total Uncertainty = √sum(error)2
113
What is the requirement for releasing a pt after a PET scan?
The dose rate at 1 m from the patient must be less than 5mR/h.
114
An air gap affects which form of radiation the most?
Protons.
For a 2 cm air gap, the Bragg peak must be moved 2 cm further into the tissue.
115
What's the attenuation er cm for a 6 MV photon beam?
3.5%
116
How does the integral whole-body dose vary between protons and XRs?
Integral whole-body dose is lower for protons, because they have no exit peak!
117
For dose calculations, 1 cm of lung tissue is equivalent to how many cms of regular tissue?
0.3
118
What is the average energy of I-125?
27 - g35 keV
119
What is the average energy of I-131?
364 keV
120
What is the dose-limiting toxicity of oral I-131 therapy?
Bone marrow toxicity, since I-131 is rapidly absorbed into the bloodstream and continuously irradiates the marrow.
121
What is the average radiation dose from an FDG PET scan?
1-4 cGy
122
What's the formula for calculating ΜU from prescription dose and isodose prescription level?
ΜU = (Rx dose) / (Isodose Rx level)
123
What's the total dose limit for a radiation worker during her pregnancy?
Total: 5 mSv
124
What's the radiation dose limit for uncontrolled areas?
0.02 mSv/wk
125
What's the radiation dose limit for controlled areas?
0.1 mSv/wk
126
What's the energy of the γ rays emitted from a PET scan?
0.511 MeV, always!
They are much more energetic than the 120-140 kVp XRs from the scanner.
127
What % deviation from normal dose qualifies as a medically reportable event?
- ≥ 20%
- Except in cases of a wrong person being treated, which should always be reported!
128
What's the formula for X-ray production efficiency?
Efficiency = electron energy (eV) x Z x 10-9
129
What's the efficiency of bremsstrahlung production for MV beams?
15-50%
130
What is the HU for water?
0
131
What are HU for lungs?
-900 to -700
Think of the density of lung tissue compared to water! Lungs are a lot of air, so they will float on water (less dense than water).
132
What are the HUs for fat?
-100 to -50
Fat is less dense than water (floats on water!)
133
What are the HUs for muscle?
+40
134
What are the HUs for soft tissue?
+100
135
What are the HUs for bone?
+700 to +1000
136
What is the **maximum** energy of a backscattered photon in a Compton interaction?
256 keV
137
What's the minimum energy required for a triplet production?
**2.04** (1.02 x 2)
138
What's the formula for **dose**?
Dose ∝ Energy/Mass
Mass energy absorption coefficient is used to calculate dose.
139
How is the mass attenuation coefficient related to the linear attenuation coefficient?
Mass attenuation coefficient = μ / ρ
140
According to TG-51, what metric is used to specify MeV e- beam quality?
R50
141
What is considered standard temperature and pressure for ionization chamber reading purposes?
- Pressure: 760 mmHg
- Temp: 22 oC
142
How do you apply temperature and pressure correction to an open ionization chamber reading?
- Temp correction:
-- convert to K (273 + temp in C)
-- correction = actual temp / 295
- Pressure correction:
-- Convert pressure to mmHg
-- correction = actual pressure / 760
143
How to calculate what % of the dose is measured by a particular ionization chamber?
The surface area of chamber/area of the beam
144
What's the formula for optical density (OD)?
OD = log (Io/It)
Io = Incident light intensity
It = transmitted light intensity
145
What is scatter maximum ratio (SMR) and how do they relate to tissue maximum ratio (TMR)?
SMR = TMR - TMRo
- SMR = Scatter component of the dose on the central beam axis
- TMRo = TMR for 0 cm field
146
What's the beam attenuation for a carbon fiber couch?
- 3%
- You need to correct either MUs or by using a model of the couch in the TPS
147
What is the integral dose?
Integral dose = mass of irradiated tissue x absorbed dose
You cannot get this from a DVH!
148
What is the mechanical tolerance for laser localization for SRS/SBRT?
± 1 mm
149
What is the tolerance for coincidence between imaging and treatment beams for SRS/SBRT?
± 1 mm
150
For a patient who received radiopharmaceuticals, when is it safe to discharge them?
When the dose to anyone will not exceed 5 mSv
151
What is the use factor (U) for secondary wall shielding?
- Always 1, since it is always receiving scattered dose.
- Therefore, U is not used in the equations.
152
What thickness of concrete is usually sufficient for primary shielding?
- 8.6 ft
- or 260 cm
153
What thickness of concrete is usually sufficient for secondary shielding?
- Usually 1/2 the primary shielding thickness
- 4.3 ft
- 130 cm
154
Which room can be used for HDR brachy w/o additional shielding?
A LINAC room, since the max energy of an Ir-192 source is 1.09 MV
155
What is the transport index?
It is the highest level of radiation (mrem/hr) at 1 m from the surface of a radiation source.
156
What is the cumulative dose limit for the lifetime of a radiation worker?
10 mSv x Age
157
How do you calculate the STD of a measurement?
STD = √measurement
158
How do you calculate the STD combination of measurements?
STD = √(STD12 + STD22...)
159
What's the typical max energy and range of protons used in the clinic?
Energy: 250 MeV
Range: 25-28 cm
160
How is dose equivalent (Sv) related to the absorbed dose?
Dose Eq = Dose x Q
- Q = quality factor
-- photons, e- = 1
-- protons, charged pions = 2
-- α particles, heavier particles = 20
-- neutrons = 5-20, depending on energy
161
What is the average energy and half-life of Y 90?
- Energy: 0.93 MeV
- Half-life: 2.67 days
162
B/w which photon energies is Compton interaction most dominant?
- PE: < 25 keV
- Compton: 25 keV - 25 MeV
- Pair production: > 25 MeV
163
What's the LET for e- at their Bragg peak?
22 MeV/cm
164
What's the average LET for e- in the tissue?
2 MeV/cm
165
What're the units of fluence?
particles/cm2
166
What's the mathematical definition of 1 cGy?
1 ergs/gram
167
What's the RBE of clinical strength protons?
1.1
168
For a SAD setup, if you know the dose at one point, 1, in the patient, how do you calculate the dose at another point, 2?
Dose ∝ TMR / r2
You need to account for both the inverse square law and tissue attenuation for SAD setups!
169
For an SSD setup, if you know the dose at one point, 1, in the patient, how do you calculate the dose at another point, 2?
Dose ∝ PDD
PDD calculated from SSD setups inherently accounts for both the inverse square law and tissue attenuation, unlike TMR for SAD setups.
170
What is the acceptable range of beam flatness?
± 3%
171
What's the formula for calculating the skin gap for an SSD/SAD setup?
Skin Gap = d/2 x (f1/SSD1 + f2/SSD2)
Can swap out SSD for SAD
172
Do AP/PA fields use SAD or SSD?
SAD, always!
173
What % 𝛥 in the linear attenuation coefficient (μ) does 1 HU represent?
0.1%
174
What's the formula for an HU?
HU = 1000 x (μmaterial-μwater)/μwater
175
What dose gradients can be achieved for multi-field IMRT plans?
10% / mm
176
What is the formula for MU calculation using the SAD setup for a square field?
MU = Dose / (K × TMR × Sc × Sp × OAF × WF × TF)
- K = Output Factor
- TMR= Tissue maximum ratio (for eqsq on the phantom/patient (**after** MLC collimation))
- Sc = Head scatter correction (for eqsq for collimator setting (**before** MLC collimation))
- Sp = Phantom scatter correction (for eqsq on the phantom/patient (**after** MLC collimation))
- OAF = Off-axis factors
- WF = Wedge Factor
- TF= Transmission factors
177
Which quantity is used to specify the strength of a brachytherapy source per the AAPM?
Air kerma strength
178
For a sealed source, what activity measured on a wipe test would be considered leaky?
- 0.005 μCi;
- 185 Bq
179
How often should a sealed source be tested for leakage?
every < 6 mos
180
What's the average energy of Cs-137?
0.662 MeV
181
How is t1/2 related to the dose rate constant?
t1/2 = 0.693/λ
λ = dose rate constant
182
How are PET isotopes produced?
Using a proton beam of energy on the order of tens of MeV.
183
Which radionuclides are produced by neutron bombardment?
198 Au, 192 Ir, 153 Sm, 125 I, 103 Pd, 89 Sr, 60 Co, and 32 P
184
Which radionuclides are produced by proton bombardment?
123-I, 68-Ga, 18-F, 15-O, 11-C
185
What does beam quality mean?
- Quality is a measure of the penetration of a beam (hence energy)
- Low-quality beams are less penetrating
- High-quality beams are more penetrating
186
How do we specify the beam quality of a kV beam (diagnostic/orthovoltage)?
Using HVL
187
How do we specify the beam quality of an MV beam (Linac)?
Using PDD
188
How do we specify the quality of an e- beam?
Using Eo, the energy of e- at the patient surface
This is always less than the maximal e- energy
189
How do we specify the beam quality of γ-ray irradiators?
Mean energy
190
What's the approx. effective energy of a poly-energetic photon beam?
1/3 x peak energy
Effective energy is the energy of a poly energetic beam with the same beam quality as the beam being measured.
191
What part of the Linac contributes to the collimator scatter, S c?
- The secondary collimator
- Primary collimator is present before the flattening filter and does not contribute much to scatter
192
How does phantom scatter, Sp, vary w/ field size and depth?
Increases w/ field size and depth
193
How often do absolute X-ray and electron LINAC outputs need to be calibrated using a water phantom?
Annually
194
What's involved in daily LINAC QA testing?
Things that directly affect delivered dose, patient, and staff safety:
- X-ray output constancy
- Laser localization
- Collimator indicators
- distance indicators
- door interlocks
195
What's the annual tolerance for x-ray and electron output calibration?
+/- 1%
196
For LINACs, How often should you check the coincidence between the light field and radiation fields?
Monthly
197
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!
198
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.
199
For an x-bit grayscale image, how many shades of gray can it represent?
2^x
200
Dose calibrations for MV photon and electron beams must be performed in what material?
In water (30 x 30 x 30 cm tank) ONLY
201
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.
202
What is the star shot QA test?
Determines radiation isocenter by exposing film to different collimator, gantry, and couch positions.
203
What is the plastic cube with a metal ball in the center QA test?
It checks the coincidence between KV and MV imaging.
204
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.
205
What are the components of IMRT gamma QA evaluation?
Dose difference and distance to agreement
206
What's the protocol for calibrating all MV therapy beams?
TG-51
207
How does the amount of bremsstrahlung production relate to e- energy, E, and atomic number, Z?
P ∝ EZ2
208
What's the T1/2 and average energy of Co-60?
- T1/2 = 5.26 yrs
- Decay rate = 1% per mo
- Average energy: γ rays w/ 1.17 and 1.33 MeV
-- For simplicity, the average 1.25 MeV is sometimes used
209
What's the average energy of e- when they are first released from the e- gun and right before they are accelerated?
50 keV
210
What's the T1/2 of Cs-131 and its average energy?
- T1/2: 9.7 d
- Average energy: γ rays w/ 0.662 MeV
211
How is the backscatter factor related to dose or treatment time?
- BSF ∝ dose
- BSF ∝ 1/time
212
How is the backscatter factor related to dose and time of treatment?
- BSF ∝ Dose
- BSF ∝ 1/ tx_time
213
What's the pitch of a helical CT scanner?
Pitch = Couch motion (cm) / slide width (cm)
If the couch moves slower than slice width, then slices will overlap.
214
What's the formula for the dose rate?
It's basically the denominator in the ΜU formula.
Dose Rate = O x PDD (or TMR) Scp x WF x TF
215
What's the value of Planck's constant?
6.63 × 10-34 joule-hertz−1
216
What's 1 amu?
- Def: 1/12th the mass of a C-12 atom
- 1.66 × 10-27 kg
- 931 MeV
217
How does penumbra vary with beam energy?
- ≤ 6 MV: ↓ penumbra w/ ↑ energy
-- Minimum penumbra at 4-6 MV
- > 6 MV: ↑ penumbra w/ ↑ energy
218
How does the e- beam output change w/ 𝛥 cutout size?
- If cutout size > range of e-, no effect on output
- If cutout size < range of e-, output changes significantly
219
What's the main purpose of a Linac QA program?
To ensure that machine characteristics do not deviate from the baseline values determined during machine install
220
To send/share DICOM images, what do you need to know about the recipient computer?
- IP address
- Port number
- application entity (AE)
221
How do you calculate workload (W) for an MV beam?
- Dose delivered at 1 m from the target per week
-- **w**orkload → **w**eekly
222
What's FEMA?
- **F**ailure **M**odes and **E**ffects **A**nalysis
- Risk-based approach to designing, evaluating, and improving the QA program
223
What's RO-ILS?
- **R**adiation **O**ncology **I**ncident **L**earning **S**ystem
- sponsored by ASTRO and AAPM
224
What's the formula for the range of protons in the tissue?
Range (cm) = 0.033E + 0.0005E2
225
What's the typical energy range of proton beams?
70-250 MeV
226
What are the units of air kerma strength?
- 1 μGy × m2 / h
- Also represented as 1U
227
What are the dose rates for LDR, MDR, and HDR brachytherapy?
- LDR - 0.4 - 2 Gy/h
- MDR: 2-12 Gy/h
- HDR: >12 Gy/h
228
How're unsealed sources given?
- Usually given systemically or injected
229
Between LDR and HDR, which technique has more normal (biological) tissue sparing?
- LDR: More normal tissue sparing 2/2 ↑ sublethal DNA damage repair
- HRD: Less normal tissue sparing 2/2 high dose rates and fx given over time shorter than that required for DNA repair
-- Geometric sparing is used to compensate for ↓ biological tissue sparing
230
What're the key dosimetric considerations for TBI?
- Uniform dose throughout the body
- Limit lung dose
- Limit dose rate (5-15 cGy/min at midplane)
231
What's the purpose of a lung block, beam spoiler, and compensator in TBI?
- Lung block reduced lung dose
- Spoiler: Increase the skin dose by increasing e- contribution to dose
- Compensator: Make the dose more homogenous throughout the body (by reducing the dose to thinner (ankles, neck, etc) parts of the body
-- Custom-designed for each patient, and can either be attached to the Linac or to the beam spoiler
232
What's the advantage of using higher beam energies for AP/PA TBI tx? How does normal tissue dose depend on pt thickness?
- ↑ energy → ↓ normal tissue dose
- ↓ thickness → ↓ normal tissue dose
233
What's the formula for calculating TBI dose homogeneity?
Dosepeak / Dosemid
234
How does dose homogeneity vary w/ beam energy, SSD, and patient thickness for a TBI tx?
- ↑ homogeneity w/
-- ↓ thickness
-- ↑ energy
-- ↑ SSD
--- ↓ PDD fall-off w/ ↑ SSD
235
What are the disadvantages of using lateral beams for a TBI tx?
- ↑ pt thickness → ↓ homogeneity
- Inability to use lung blocks
-- rely on pt's arms to reduce lung dose
236
After HDR source exchange, the activity of the source should be within what % of the manufacturer's certificate?
±3%
237
How does the dose at a point vary w/ distance, r, for a line source?
Dose ∝ 1/r
238
What's the max allowable deviation between measured and intended dwell positions and step-size spacing b/w dwell positions?
±1mm
239
For which 2 LDR sources does the inverse square (1/r2) law not hold within ± 5 cm?
- Sources
-- Pd-103
-- I-125
- Reason?
-- Due to their low energy, their attenuation is much larger than the scatter, so it does not cancel out.
-- For other sources, their attenuation (↓ dose) and scatter (↑ dose) are equal within the first 5 cm, so they follow the 1/r2 law
240
What's the formula for Risk Priority Number?
- RPN = S × D × O
-- Risk Priority Number
-- S = Severity (1-10 range)
-- D = Detectability (1-10 range, 1 is easy to detect, 10 is hard to detect)
-- O = Occurrence
241
What's the formula for calculating air kerma strength of a brachytherapy source?
- Sk = K × l2
-- Sk = Air kerma strength
-- K = Air kerma rate
-- l = distance
- Units = μGy m2h-1
242
What're the units of specific activity?
- mCi/g
- High specific activity helps the fabrication of small sources
243
What're the SI units of brachytherapy source activity?
Bq
244
How does the probability of photonuclear disintegration vary w/ atomic #?
- increases w/ atomic number
- Lead (high Z) is an infamous source of neutrons
244
What's the formula for the required barrier thickness?
- B = P × d2 / WUT
-- B = barrier transmission factor
-- P = permissible dose
-- d = distance from source
-- W = **w**orkload, total **w**eekly radiation
-- U = Use factor,
245
What's the formula for the required barrier thickness?
- B = P × d2 / WUT
-- B = barrier transmission factor
-- P = permissible dose
-- d = distance from source
-- W = **w**orkload, total **w**eekly radiation
-- U = Use factor, fx of operating time during which a Linac is directed towards a particular barrier
-- T =
245
What's the typical proton range uncertainty?
2% - 5%
246
What's the formula for the required barrier thickness?
- B = P × d2 / WUT
-- B = barrier transmission factor
-- P = permissible dose
-- d = distance from the source
-- W = **w**orkload, total **w**eekly radiation
-- U = Use factor, fx of operating time during which a Linac is directed towards a particular barrier
-- T = occupancy factor; fx of operating time during which the area is occupied
