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RadPhys > Key Facts + Eq > Flashcards

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

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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
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3
Q

What’s the TMR for a 10x10 cm2 6 MV photon beam at 10 cm?

A

0.784

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4
Q

What’s the PDD for a 10x10 cm2 6MV photon beam at 10 cm?

A

~68%

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5
Q

What’s the average energy of Co-60?

A

1.25 MeV

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6
Q

What’s the formula for inverse square factor?

A

ISF = [SSDref + dmax/SSD + dmax]2

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7
Q

What are Sc and Sp (used in MU calculations) for a 10x10 field?

A

1

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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

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9
Q

What’s the equation relating the decay constant, γ, to t1/2?

A

t1/2 = ln2/γ

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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
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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

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12
Q

What is the conversion from Tesla (SI Unit) to Gauss (CGS units)?

A

Both measure magnetic flux (strength)

1T = 10,000 G

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13
Q

What is the equivalent square of a circular field of radius r?

A

EqSq = r√ π

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14
Q

How do you calculate TVL from μ or HVL?

A
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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
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16
Q

The formula for HVL?

A

HVL = ln2/µ

ln2 = 0.693

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17
Q

What is ln2?

A

0.693

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18
Q

What is the conversion between Curie (Ci) and becquerel (Bq)?

A

1 Ci = 37 x 109 Bq

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19
Q

What is fluence (Φ)?

A

Number of particles (N) incident on a sphere of cross-sectional area A:

Φ = N/A

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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

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21
Q

What’s the t1/2 of Co-60?

A

5.26 yrs

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22
Q

Dmax of a Co 60 beam?

A

0.5 cm

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23
Q

Dmax of a 18 MV photon beam?

A

3.5 cm

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24
Q

Dmax of a 15 MV photon beam?

A

3.0 cm

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25
Q

What is the dmax of a 10MV photon beam?

A

2.5 cm

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26
Q

What is the dmax of a 6MV photon beam?

A

1.5 cm

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27
Q

What is the total dose (TD) delivered by a permanent brachytherapy implant of half-life, t1/2, and initial dose rate?

A

First, calculate the mean life (ML) of an isotope

  • ML = 1/γ
  • ML = 1.44 x t1/2
  • TD = ML x Initial Dose Rate
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28
Q

For an x-bit grayscale image, how many shades of gray can it represent?

A

2x

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29
Q

What is the conversion from bit to byte?

A

8 bit = 1 byte

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30
Q

What is the conversion from byte to kilobyle?

A

1 kilobyte = 1024 bytes

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31
Q

What is the conversion from kilobyte to megabyte?

A

1 megabyte = 1024 kilobytes

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32
Q

How are intensities (I) related to distances (r) from a radiation source?

A

(I1/I2) = (r2/r1)^2

It’s the inverse square law!

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33
Q

What is the equation of the XR output of an XR tube?

A

Output = tube current x exposure time x kVp^2

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34
Q

What is the mean energy of I-125?

A

35.5 keV

Mnemonic: it is ~ half it’s half life (60 days)

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35
Q

How does I-125 decay?

A

Decays via electron capture and subsequent gamma release.

  • e- caputure: 125I → 125Te
  • Decay back to ground state by emitting 35.5 keV γ ray
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36
Q

What is the HVL of I-125 in lead?

A

0.025 mm

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37
Q

How does Pd-103 decay?

A

It decays via electron capture and emission of gamma rays, just like I-125.

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38
Q

What’s the t1/2 of Pd-103?

A

17 days

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39
Q

What is the average radiation energy of Pd-103?

A

21 keV

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40
Q

What is the HVL of Pd-103 in lead?

A

0.0085 mm

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41
Q

What is the average energy and t1/2 of Ir-192?

A
  • 72 days
  • 0.38 MeV γ

Higher than other sources, which makes sense since it is used for HDR.

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42
Q

How does Ir-192 decay?

A

It decays via B minus and gamma emission.

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43
Q

How does Cs-137 decay?

A

It decays via B minus and gamma emission.

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44
Q

What’s the t1/2 of Cs-137?

A

30 yrs

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45
Q

What’s the t1/2 of Cs-131?

A

9.7 days

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46
Q

What’s the average energy of Cs-131?

A

30 keV

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47
Q

How does Cs-131 decay?

A

e- capture and XR emission

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48
Q

What is the average radiation energy of Cs-137?

A

0.662 MeV

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49
Q

What is the HVL of Cs-137 in lead?

A

5.5 mm

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50
Q

What is the HVL or Ir-192 in lead?

A

2.5 mm

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51
Q

What is the maximum number of electrons that can fit into a shell?

A

2n2

n = shell number

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52
Q

What is the range (in cm) of electrons in water and air?

A

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

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53
Q

What is the formula for various isodose lines for electrons of incident energy Eo?

A
  • 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!)

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54
Q

What is the dmax of a 4 MV photon beam?

A

1 cm

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55
Q

What is the average public radiation exposure for adults?

A

Daily: 0.017 mSv per day
Annual: 6.2 mSv per yr

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56
Q

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?

A

48%

CT - 24%
Nuc Med - 17%
Fluoroscopy - 7%

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57
Q

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?

A

Radon - 37%
Cosmic - 5%
Consumer radiation exposure - 2%

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58
Q

What is the annual NCRP dose limit for occupation radiation exposure for adults?

A

50 mSv

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59
Q

What is the NRCP effective dose limit for the general public?

A

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.

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60
Q

What is the NCRP radiation dose limit for the fetus of a pregnant radiation worker?

A

5 mSv/year

1/10 the limit to a non-pregnant worker

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61
Q

What is the rest mass of an electron or a positron?

A

0.511 MeV

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62
Q

What is the minimum amount of photon energy required for pair production?

A

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.

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63
Q

What is the density of water?

A

1 g/cm3

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64
Q

What is the formula for the effective depth of a beam as it passes through inhomogeneous tissues?

A

Dep_eff = (dep_1den_1) + (dep_2den_2)…

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65
Q

What is the formula for the amount of shielding required for a radiation bunker?

A

Shielding ∝ (workload x use factor x occupancy) / distance2

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66
Q

How many mm of Cerrobend relative to lead are required to achieve the same shielding?

A
  • 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
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67
Q

What attenuation per mm of lead (shielding/blocking) completely blocks an electron beam relative to beam energy?

A

2 MeV electron energy is completely blocked by every mm of lead.

Therefore, beam energy/2 (+1mm) is the length of lead block required.

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68
Q

What is the formula for calculating the optimal wedge angle?

A

Wedge angle = 90 - (hinge angle/2)

Hinge angle = gantry angle

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69
Q

How are the wedges arranged for opposed tangents for breast treatments?

A

The heels of the wedges face towards the anterior (nipple) part of the breast.

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70
Q

What is the thicker part of a wedge called?

A

The heel

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71
Q

What is the thinner part of the wedge called?

A

The toe

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72
Q

Under which part, the heel or the toe, is the dose distribution hot vs. cold?

A

Under the heel, the dose distribution is cold (more attenuation). Under the toe, the dose distribution is hot (less attenuation).

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73
Q

How often should the department QA committee meet?

A

At least monthly

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74
Q

How can you increase the maximum field length without increasing SSD?

A

Rotate the collimator by 45 deg.

The size would be √2* (f)
Where f is the field size.

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75
Q

Conversion from eV to J?

A

1.6 x 10^=10 J

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76
Q

Rest mass of the proton in eV?

A

938 MeV

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77
Q

The formula for effective half-life?

A
  • 1/Teff = 1/Trad + 1/Tbio
  • Teff < Trad or Tbiof
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78
Q

The formula for power?

A

Power = V x A

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79
Q

What is the Mayneord F-factor and what is the formula for calculating it?

A

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

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80
Q

What is the relationship between TVL and transmission?

A
  • log (desired transmission/current transmission) * TVL
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81
Q

The dosimeter to measure fetal dose in a pregnant radiation worker is placed where? Does it measure daily, weekly, monthly, or yearly dose?

A

It’s placed around the abdomen. It measure monthly dose (limit ≤ 50 mrem)

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82
Q

How should you orient the MLCs if you want to minimize head leakage?

A

90 (leaves travel parallel to the sup/inf axis of the patient

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83
Q

What does V mean in dose constraints?

A

V = volume receiving a dose ≥ X Gy

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84
Q

What does D mean in dose constraints?

A

The dose received by X % of the volume

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85
Q

What is the conversion between R and cGy?

A

1 R = 0.876 cGy

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86
Q

What is the conversion between Sv and rem?

A

1 Sv = 100 rem

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87
Q

The formula for MU calc for electron beams?

A

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)

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88
Q

What’s the formula for PDD?

A

(Dose rate at depth / dose rate at dmax) x 100

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89
Q

The formula for the practical range (R_p) of electrons?

A

E/2

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90
Q

The formula for the range of 80% IDL (R_80) of electrons?

A

E/2.8

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91
Q

The formula for the 90%V IDL (R_90) of electrons?

A

E/3.3

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92
Q

What is the output factor (K) for an electron beam at d_max and 100 cm SSD?

A

1

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93
Q

What is the ISF for electron beams?

A

ISF = (SSDeff / (SSDeff + ΔSSD))^2

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94
Q

What’s the formula for calculating CT numbers (Hounsfield units)?

A

CT number = 1000 x [{μ_mat - μ_water)/μ_water]

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95
Q

How does the range of e relate to electron density?

A

Higher the e-density, the shorter the range
e density is proportional to mass density (Hounsfield Units)

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96
Q

What’s the beam energy for a tomotherapy unit?

A

6 MV only

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97
Q

What’s the % attenuation for 6 MV and 15 MV photon beams?

A
  • 6 MV: 3%/cm
  • 15 MV: 2%/cm
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98
Q

What’s the air kerma rate at the pubic symphysis for patients receiving I-125 seed implantation in the prostate?

A

25 μGy/h

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99
Q

What thickness of concrete is enough to shield Linacs outputting up to 18 MV of photons?

A

260 cm, or 8.6 ft

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100
Q

How much higher are the linac-leakage workloads for IMRT vs. conventional radiation?

A

2-10 x higher

101
Q

What’s the formula for the required barrier thickness?

A
  • B = P × d2 / WUT
    – B = barrier transmission factor
    – P = permissible dose
    – d = distance from the source
    – W = Workload, total weekly 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
Q

What is the formula for permissible dose equivalent for an area?

A

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
Q

At which energies is the PE effect the most dominant interaction?

A
  • dominant: 10 - 25 keV
  • range: 1-150 keV
104
Q

What does the probability of the PE effect depend on?

A

PE probability ∝ Z3/E3

105
Q

At which energies is Compton scattering the most dominant interaction?

A
  • dominant: 26 kEv - 24 MeV
  • range: any
106
Q

At which energies is pair production the most dominant interaction?

A
  • dominant: > 10 MeV
  • range: 1.02 MeV and above
107
Q

What does the probability of the pair production depend on?

A

Probability is proportional to Z2 and increases dramatically w/ energy.

108
Q

What’s the neutron leakage for 18 MV photons?

A

0.15%

109
Q

What’s the neutron leakage for 10 MV photons?

A

0.04%

110
Q

What’s the t1/2 and average energy of Ra-226?

A
  • 1601 yrs
  • 0.83 MeV γ
111
Q

What’s the t1/2 and average energy of Rn-222?

A
  • 2.7 days
  • 0.83 MeV γ
112
Q

How do you calculate the total uncertainty of a treatment delivery setup?

A

Total Uncertainty = √sum(error)2

113
Q

What is the requirement for releasing a pt after a PET scan?

A

The dose rate at 1 m from the patient must be less than 5mR/h.

114
Q

An air gap affects which form of radiation the most?

A

Protons.

For a 2 cm air gap, the Bragg peak must be moved 2 cm further into the tissue.

115
Q

What’s the attenuation er cm for a 6 MV photon beam?

A

3.5%

116
Q

How does the integral whole-body dose vary between protons and XRs?

A

Integral whole-body dose is lower for protons, because they have no exit peak!

117
Q

For dose calculations, 1 cm of lung tissue is equivalent to how many cms of regular tissue?

A

0.3

118
Q

What is the average energy of I-125?

A

27 - g35 keV

119
Q

What is the average energy of I-131?

A

364 keV

120
Q

What is the dose-limiting toxicity of oral I-131 therapy?

A

Bone marrow toxicity, since I-131 is rapidly absorbed into the bloodstream and continuously irradiates the marrow.

121
Q

What is the average radiation dose from an FDG PET scan?

A

1-4 cGy

122
Q

What’s the formula for calculating ΜU from prescription dose and isodose prescription level?

A

ΜU = (Rx dose) / (Isodose Rx level)

123
Q

What’s the total dose limit for a radiation worker during her pregnancy?

A

Total: 5 mSv

124
Q

What’s the radiation dose limit for uncontrolled areas?

A

0.02 mSv/wk

125
Q

What’s the radiation dose limit for controlled areas?

A

0.1 mSv/wk

126
Q

What’s the energy of the γ rays emitted from a PET scan?

A

0.511 MeV, always!

They are much more energetic than the 120-140 kVp XRs from the scanner.

127
Q

What % deviation from normal dose qualifies as a medically reportable event?

A
  • ≥ 20%
  • Except in cases of a wrong person being treated, which should always be reported!
128
Q

What’s the formula for X-ray production efficiency?

A

Efficiency = electron energy (eV) x Z x 10-9

129
Q

What’s the efficiency of bremsstrahlung production for MV beams?

A

15-50%

130
Q

What is the HU for water?

A

0

131
Q

What are HU for lungs?

A

-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
Q

What are the HUs for fat?

A

-100 to -50

Fat is less dense than water (floats on water!)

133
Q

What are the HUs for muscle?

A

+40

134
Q

What are the HUs for soft tissue?

A

+100

135
Q

What are the HUs for bone?

A

+700 to +1000

136
Q

What is the maximum energy of a backscattered photon in a Compton interaction?

A

256 keV

137
Q

What’s the minimum energy required for a triplet production?

A

2.04 (1.02 x 2)

138
Q

What’s the formula for dose?

A

Dose ∝ Energy/Mass

Mass energy absorption coefficient is used to calculate dose.

139
Q

How is the mass attenuation coefficient related to the linear attenuation coefficient?

A

Mass attenuation coefficient = μ / ρ

140
Q

According to TG-51, what metric is used to specify MeV e- beam quality?

A

R50

141
Q

What is considered standard temperature and pressure for ionization chamber reading purposes?

A
  • Pressure: 760 mmHg
  • Temp: 22 oC
142
Q

How do you apply temperature and pressure correction to an open ionization chamber reading?

A
  • Temp correction:
    – convert to K (273 + temp in C)
    – correction = actual temp / 295
  • Pressure correction:
    – Convert pressure to mmHg
    – correction = actual pressure / 760
143
Q

How to calculate what % of the dose is measured by a particular ionization chamber?

A

The surface area of chamber/area of the beam

144
Q

What’s the formula for optical density (OD)?

A

OD = log (Io/It)

Io = Incident light intensity
It = transmitted light intensity

145
Q

What is scatter maximum ratio (SMR) and how do they relate to tissue maximum ratio (TMR)?

A

SMR = TMR - TMRo

  • SMR = Scatter component of the dose on the central beam axis
  • TMRo = TMR for 0 cm field
146
Q

What’s the beam attenuation for a carbon fiber couch?

A
  • 3%
  • You need to correct either MUs or by using a model of the couch in the TPS
147
Q

What is the integral dose?

A

Integral dose = mass of irradiated tissue x absorbed dose

You cannot get this from a DVH!

148
Q

What is the mechanical tolerance for laser localization for SRS/SBRT?

A

± 1 mm

149
Q

What is the tolerance for coincidence between imaging and treatment beams for SRS/SBRT?

A

± 1 mm

150
Q

For a patient who received radiopharmaceuticals, when is it safe to discharge them?

A

When the dose to anyone will not exceed 5 mSv

151
Q

What is the use factor (U) for secondary wall shielding?

A
  • Always 1, since it is always receiving scattered dose.
  • Therefore, U is not used in the equations.
152
Q

What thickness of concrete is usually sufficient for primary shielding?

A
  • 8.6 ft
  • or 260 cm
153
Q

What thickness of concrete is usually sufficient for secondary shielding?

A
  • Usually 1/2 the primary shielding thickness
  • 4.3 ft
  • 130 cm
154
Q

Which room can be used for HDR brachy w/o additional shielding?

A

A LINAC room, since the max energy of an Ir-192 source is 1.09 MV

155
Q

What is the transport index?

A

It is the highest level of radiation (mrem/hr) at 1 m from the surface of a radiation source.

156
Q

What is the cumulative dose limit for the lifetime of a radiation worker?

A

10 mSv x Age

157
Q

How do you calculate the STD of a measurement?

A

STD = √measurement

158
Q

How do you calculate the STD combination of measurements?

A

STD = √(STD12 + STD22…)

159
Q

What’s the typical max energy and range of protons used in the clinic?

A

Energy: 250 MeV
Range: 25-28 cm

160
Q

How is dose equivalent (Sv) related to the absorbed dose?

A

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
Q

What is the average energy and half-life of Y 90?

A
  • Energy: 0.93 MeV
  • Half-life: 2.67 days
162
Q

B/w which photon energies is Compton interaction most dominant?

A
  • PE: < 25 keV
  • Compton: 25 keV - 25 MeV
  • Pair production: > 25 MeV
163
Q

What’s the LET for e- at their Bragg peak?

A

22 MeV/cm

164
Q

What’s the average LET for e- in the tissue?

A

2 MeV/cm

165
Q

What’re the units of fluence?

A

particles/cm2

166
Q

What’s the mathematical definition of 1 cGy?

A

1 ergs/gram

167
Q

What’s the RBE of clinical strength protons?

A

1.1

168
Q

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?

A

Dose ∝ TMR / r2

You need to account for both the inverse square law and tissue attenuation for SAD setups!

169
Q

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?

A

Dose ∝ PDD

PDD calculated from SSD setups inherently accounts for both the inverse square law and tissue attenuation, unlike TMR for SAD setups.

170
Q

What is the acceptable range of beam flatness?

A

± 3%

171
Q

What’s the formula for calculating the skin gap for an SSD/SAD setup?

A

Skin Gap = d/2 x (f1/SSD1 + f2/SSD2)

Can swap out SSD for SAD

172
Q

Do AP/PA fields use SAD or SSD?

A

SAD, always!

173
Q

What % 𝛥 in the linear attenuation coefficient (μ) does 1 HU represent?

A

0.1%

174
Q

What’s the formula for an HU?

A

HU = 1000 x (μmaterial-μwater)/μwater

175
Q

What dose gradients can be achieved for multi-field IMRT plans?

A

10% / mm

176
Q

What is the formula for MU calculation using the SAD setup for a square field?

A

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
Q

Which quantity is used to specify the strength of a brachytherapy source per the AAPM?

A

Air kerma strength

178
Q

For a sealed source, what activity measured on a wipe test would be considered leaky?

A
  • 0.005 μCi;
  • 185 Bq
179
Q

How often should a sealed source be tested for leakage?

A

every < 6 mos

180
Q

What’s the average energy of Cs-137?

A

0.662 MeV

181
Q

How is t1/2 related to the dose rate constant?

A

t1/2 = 0.693/λ

λ = dose rate constant

182
Q

How are PET isotopes produced?

A

Using a proton beam of energy on the order of tens of MeV.

183
Q

Which radionuclides are produced by neutron bombardment?

A

198 Au, 192 Ir, 153 Sm, 125 I, 103 Pd, 89 Sr, 60 Co, and 32 P

184
Q

Which radionuclides are produced by proton bombardment?

A

123-I, 68-Ga, 18-F, 15-O, 11-C

185
Q

What does beam quality mean?

A
  • 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
Q

How do we specify the beam quality of a kV beam (diagnostic/orthovoltage)?

A

Using HVL

187
Q

How do we specify the beam quality of an MV beam (Linac)?

A

Using PDD

188
Q

How do we specify the quality of an e- beam?

A

Using Eo, the energy of e- at the patient surface

This is always less than the maximal e- energy

189
Q

How do we specify the beam quality of γ-ray irradiators?

A

Mean energy

190
Q

What’s the approx. effective energy of a poly-energetic photon beam?

A

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
Q

What part of the Linac contributes to the collimator scatter, S c?

A
  • The secondary collimator
  • Primary collimator is present before the flattening filter and does not contribute much to scatter
192
Q

How does phantom scatter, Sp, vary w/ field size and depth?

A

Increases w/ field size and depth

193
Q

How often do absolute X-ray and electron LINAC outputs need to be calibrated using a water phantom?

A

Annually

194
Q

What’s involved in daily LINAC QA testing?

A

Things that directly affect delivered dose, patient, and staff safety:
- X-ray output constancy
- Laser localization
- Collimator indicators
- distance indicators
- door interlocks

195
Q

What’s the annual tolerance for x-ray and electron output calibration?

A

+/- 1%

196
Q

For LINACs, How often should you check the coincidence between the light field and radiation fields?

A

Monthly

197
Q

For LINACs, how often should you check the coincidence between kV imaging isocenter and MV treatment isocenter?

A

Daily!

Variations in this can directly affect patient setup integrity!

198
Q

What defines the QA tolerances for linear accelerators?

A

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
Q

For an x-bit grayscale image, how many shades of gray can it represent?

A

2^x

200
Q

Dose calibrations for MV photon and electron beams must be performed in what material?

A

In water (30 x 30 x 30 cm tank) ONLY

201
Q

What is the Winston-Lutz QA method? What does it test?

A

It is MV imaging to check the coincidence between the gantry isocenter and laser alignment.

202
Q

What is the star shot QA test?

A

Determines radiation isocenter by exposing film to different collimator, gantry, and couch positions.

203
Q

What is the plastic cube with a metal ball in the center QA test?

A

It checks the coincidence between KV and MV imaging.

204
Q

What is the end-to-end test and what does it test?

A

It carries out a treatment plan in a phantom (sim, plan, and treat the phantom). It needs to occur yearly.

205
Q

What are the components of IMRT gamma QA evaluation?

A

Dose difference and distance to agreement

206
Q

What’s the protocol for calibrating all MV therapy beams?

A

TG-51

207
Q

How does the amount of bremsstrahlung production relate to e- energy, E, and atomic number, Z?

A

P ∝ EZ2

208
Q

What’s the T1/2 and average energy of Co-60?

A
  • 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
Q

What’s the average energy of e- when they are first released from the e- gun and right before they are accelerated?

A

50 keV

210
Q

What’s the T1/2 of Cs-131 and its average energy?

A
  • T1/2: 9.7 d
  • Average energy: γ rays w/ 0.662 MeV
211
Q

How is the backscatter factor related to dose or treatment time?

A
  • BSF ∝ dose
  • BSF ∝ 1/time
212
Q

How is the backscatter factor related to dose and time of treatment?

A
  • BSF ∝ Dose
  • BSF ∝ 1/ tx_time
213
Q

What’s the pitch of a helical CT scanner?

A

Pitch = Couch motion (cm) / slide width (cm)

If the couch moves slower than slice width, then slices will overlap.

214
Q

What’s the formula for the dose rate?

A

It’s basically the denominator in the ΜU formula.

Dose Rate = O x PDD (or TMR) Scp x WF x TF

215
Q

What’s the value of Planck’s constant?

A

6.63 × 10-34 joule-hertz−1

216
Q

What’s 1 amu?

A
  • Def: 1/12th the mass of a C-12 atom
  • 1.66 × 10-27 kg
  • 931 MeV
217
Q

How does penumbra vary with beam energy?

A
  • ≤ 6 MV: ↓ penumbra w/ ↑ energy
    – Minimum penumbra at 4-6 MV
  • > 6 MV: ↑ penumbra w/ ↑ energy
218
Q

How does the e- beam output change w/ 𝛥 cutout size?

A
  • If cutout size > range of e-, no effect on output
  • If cutout size < range of e-, output changes significantly
219
Q

What’s the main purpose of a Linac QA program?

A

To ensure that machine characteristics do not deviate from the baseline values determined during machine install

220
Q

To send/share DICOM images, what do you need to know about the recipient computer?

A
  • IP address
  • Port number
  • application entity (AE)
221
Q

How do you calculate workload (W) for an MV beam?

A
  • Dose delivered at 1 m from the target per week
    – workload → weekly
222
Q

What’s FEMA?

A
  • Failure Modes and Effects Analysis
  • Risk-based approach to designing, evaluating, and improving the QA program
223
Q

What’s RO-ILS?

A
  • Radiation Oncology Incident Learning System
  • sponsored by ASTRO and AAPM
224
Q

What’s the formula for the range of protons in the tissue?

A

Range (cm) = 0.033E + 0.0005E2

225
Q

What’s the typical energy range of proton beams?

A

70-250 MeV

226
Q

What are the units of air kerma strength?

A
  • 1 μGy × m2 / h
  • Also represented as 1U
227
Q

What are the dose rates for LDR, MDR, and HDR brachytherapy?

A
  • LDR - 0.4 - 2 Gy/h
  • MDR: 2-12 Gy/h
  • HDR: >12 Gy/h
228
Q

How’re unsealed sources given?

A
  • Usually given systemically or injected
229
Q

Between LDR and HDR, which technique has more normal (biological) tissue sparing?

A
  • 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
Q

What’re the key dosimetric considerations for TBI?

A
  • Uniform dose throughout the body
  • Limit lung dose
  • Limit dose rate (5-15 cGy/min at midplane)
231
Q

What’s the purpose of a lung block, beam spoiler, and compensator in TBI?

A
  • 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
Q

What’s the advantage of using higher beam energies for AP/PA TBI tx? How does normal tissue dose depend on pt thickness?

A
  • ↑ energy → ↓ normal tissue dose
  • ↓ thickness → ↓ normal tissue dose
233
Q

What’s the formula for calculating TBI dose homogeneity?

A

Dosepeak / Dosemid

234
Q

How does dose homogeneity vary w/ beam energy, SSD, and patient thickness for a TBI tx?

A
  • ↑ homogeneity w/
    – ↓ thickness
    – ↑ energy
    – ↑ SSD
    — ↓ PDD fall-off w/ ↑ SSD
235
Q

What are the disadvantages of using lateral beams for a TBI tx?

A
  • ↑ pt thickness → ↓ homogeneity
  • Inability to use lung blocks
    – rely on pt’s arms to reduce lung dose
236
Q

After HDR source exchange, the activity of the source should be within what % of the manufacturer’s certificate?

A

±3%

237
Q

How does the dose at a point vary w/ distance, r, for a line source?

A

Dose ∝ 1/r

238
Q

What’s the max allowable deviation between measured and intended dwell positions and step-size spacing b/w dwell positions?

A

±1mm

239
Q

For which 2 LDR sources does the inverse square (1/r2) law not hold within ± 5 cm?

A
  • 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
Q

What’s the formula for Risk Priority Number?

A
  • 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
Q

What’s the formula for calculating air kerma strength of a brachytherapy source?

A
  • Sk = K × l2
    – Sk = Air kerma strength
    – K = Air kerma rate
    – l = distance
  • Units = μGy m2h-1
242
Q

What’re the units of specific activity?

A
  • mCi/g
  • High specific activity helps the fabrication of small sources
243
Q

What’re the SI units of brachytherapy source activity?

A

Bq

244
Q

How does the probability of photonuclear disintegration vary w/ atomic #?

A
  • increases w/ atomic number
  • Lead (high Z) is an infamous source of neutrons
244
Q

What’s the formula for the required barrier thickness?

A
  • B = P × d2 / WUT
    – B = barrier transmission factor
    – P = permissible dose
    – d = distance from source
    – W = workload, total weekly radiation
    – U = Use factor,
245
Q

What’s the formula for the required barrier thickness?

A
  • B = P × d2 / WUT
    – B = barrier transmission factor
    – P = permissible dose
    – d = distance from source
    – W = workload, total weekly radiation
    – U = Use factor, fx of operating time during which a Linac is directed towards a particular barrier
    – T =
245
Q

What’s the typical proton range uncertainty?

A

2% - 5%

246
Q

What’s the formula for the required barrier thickness?

A
  • B = P × d2 / WUT
    – B = barrier transmission factor
    – P = permissible dose
    – d = distance from the source
    – W = workload, total weekly 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

RadPhys (10 decks)

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