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Residency > First Year Exam: Rules of Thumb and Values to Memorize > Flashcards

First Year Exam: Rules of Thumb and Values to Memorize Flashcards

1
Q

What is the approximate pdd(10) for a 6MV beam?

A

65-67%

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2
Q

What is the approximate pdd(10) for a 6FFF beam?

A

63-64%

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3
Q

What is the approximate pdd(10) for a 10MV beam?

A

73-75%

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4
Q

What is the approximate pdd(10) for a 10FFF beam?

A

70-71%

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5
Q

What is the approximate pdd(10) for a 15MV beam?

A

76-77%

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6
Q

What is the approximate pdd(10) for a 18MV beam?

A

80%

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7
Q

What is the approximate dmax for a 6FFF beam?

A

1.4 - 1.5 cm

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8
Q

What is the approximate dmax for a 6MV beam?

A

1.4 - 1.5 cm

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9
Q

What is the approximate dmax for a 10FFF beam?

A

2.2 - 2.4 cm

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10
Q

What is the approximate dmax for a 10MV beam?

A

2.2 - 2.4 cm

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11
Q

What is the approximate dmax for a 15MV beam?

A

2.7 - 2.9 cm

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12
Q

What is the approximate dmax for a 18MV beam?

A

3.3 cm

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13
Q

What is the approximate dmax for a Co-60 beam?

A

0.5 cm

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14
Q

What is the approximate pdd(10) for a Co-60 beam?

A

55%

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15
Q

What is the attenuation rate dose fall-off rule of thumb for 6MV beams past dmax?

A

3%/cm

NOTE: This is just attenuation. The actual fall-off rate differs because it’s also inverse square dependent, and the inverse square fall-off varies with distance from source. So this 3%/cm you can only use for TMR, but it’s not as applicable for PDD. For PDD, expect something larger than 3%/cm

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16
Q

What is the attenuation rate dose fall-off rule of thumb for 10MV beams past dmax?

A

2.5%/cm

NOTE: This is just attenuation. The actual fall-off rate differs because it’s also inverse square dependent, and the inverse square fall-off varies with distance from source. So this 2.5%/cm you can only use for TMR, but it’s not as applicable for PDD. For PDD, expect something larger than 2.5%/cm

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17
Q

What is the attenuation rate dose fall-off rule of thumb for 18MV beams past dmax?

A

2%/cm

NOTE: This is just attenuation. The actual fall-off rate differs because it’s also inverse square dependent, and the inverse square fall-off varies with distance from source. So this 2%/cm you can only use for TMR, but it’s not as applicable for PDD. For PDD, expect something larger than 2%/cm

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18
Q

What is the average energy of a photon beam?

A

Max energy / 3

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19
Q

How is a change in flatness indicative of a change in energy?

A

A 10% change in flatness corresponds to a 1% change in energy

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20
Q

What is the average energy of an electron beam at patient surface?

A

Equal to Emax (since electron beams are close to monoenergetic)

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21
Q

If given E0, what is the approximate R50 in water?

A

R50(cm) = E0/2.33

NOTE: This is actually an overestimation. The actual measurement of R50 is typically a bit below what this equation predicts

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22
Q

If given E0, what is the approximate R80 in water?

A

R80(cm) = E0/2.9

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23
Q

If given E0, what is the approximate R90 in water?

A

R90(cm) = E0/3.2

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24
Q

If given E0, what is the approximate Rp in water?

A

Rp(cm) = E0/2

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25
What is approximate bremmstrahlung tail for 6E, 12E and 20E respectively?
6E - 1% 12E - 2% 20E - 5%
26
What is approximate dmax of a 6E beam?
1.2 - 1.3 cm
27
What is approximate dmax of a 9E beam?
1.9 - 2.1 cm
28
What is approximate dmax of a 12E beam?
2.8 - 2.9 cm
29
What is approximate dmax of a 16E beam?
3.0 - 3.1 cm
30
What is approximate dmax of a 20E beam?
2.5 cm
31
What is the approximate surface dose for electrons with energies between 6E and 20E, and how does it change with increasing energy?
78 - 95% Increases with increasing energy
32
What is the approximate surface dose for photons with energies between Co-60 and 20MV, and how does it change with increasing energy?
20 - 50% Decreases with increasing energy
33
What is the equivalent square field equation?
A/P = A/P or... x^2/4x = AB/2(A+B) Solve for x
34
What is approximate scatter at 1 meter from a phantom?
1/1000th the primary beam
35
What is the maximum energy of lateral photon scatter?
0.511 MeV
36
What is the maximum energy of backscattering photons?
0.255 MeV
37
How are the Compton, PE, and PP mass attenuation coefficients proportional to Z?
Compton mass atten - independent of Z, proportional to electron-density PE mass atten - proportional to Z^3 PP mass atten - proportional to Z
38
What isodose line is LINAC based SRS usually prescribed to? What about Gamma Knife?
LINAC: 80% line GK: 50% line
39
What is approximate scatter outside of field as a percentage of central axis dose for physical and dynamic wedges?
Physical: ~2.5% of central axis dose Dynamic: ~1% of central axis dose
40
What is the approximate R50 of a 6E beam?
2.3 - 2.4 cm
41
What is the approximate R50 of a 9E beam?
3.5 - 3.6 cm
42
What is the approximate R50 of a 12E beam?
5 cm
43
What is the approximate R50 of a 16E beam?
6.7 cm
44
What is the approximate R50 of a 20E beam?
8.3 - 8.5 cm
45
What is the general rule of thumb for lead shielding needed for electron machine of max energy E?
Thickness of lead (mm) = E(MeV) / 2 Or sometimes I've seen Thickness of lead (mm) = Thickness of water / 11
46
Rate of electron beam attenuation in lead
2 MeV/mm
47
What thickness of cerrobend is needed to stop an electron beam?
Thickness in mm = 1.2*thickness in lead
48
To estimate margin for PTV, what is the equation used?
margin = 2.5ε + 0.7σ Where ε is systematic uncertainty (setup, motion, imaging, etc) (possible to reduce with IGRT) σ is random uncertainty (very hard to reduce)
49
What is the approximate therapeutic range of an electron beam given E?
Therapeutic range ~ E/3 cm
50
Given energy of an electron beam, what equation approximates surface dose?
E + 73% Or E + 72%
51
What is the speed of sound in soft tissue?
1540 m/s
52
Approximately how much dose do you receive from a CT scan?
50 - 75 mGy
53
What is the approximate HU of air?
-1000
54
What is the approximate HU of lung?
-700
55
What is the approximate HU of soft tissue?
-300 to -100
56
What is the approximate HU of fat?
-80
57
What is the approximate HU of water?
0
58
What is the approximate HU of CSF?
15
59
What is the approximate HU of blood and muscle?
40
60
What is the approximate HU of spongy bone?
400+
61
What is the approximate HU of cortical bone?
up to 3000
62
In a T1 weighted MRI, what appears bright and what appears dark?
Fat is bright, CSF and water are dark
63
In a T2 weighted MRI, what appears bright and what appears dark?
Bone is dark, water and CSF are bright
64
In a FLAIR image, what appears bright and what appears dark?
FLAIR is same as T1, except all fluid signal is removed So fat is bright, CSF and water are dark
65
What is a typical MLC intra-leaf leakage?
1% to 2%
66
What is a typical MLC inter-leaf leakage?
2% to 4%
67
What is a typical MLF leaf-end leakage?
15% to 24%
68
What is the approximate Z of air?
7.6
69
What is the approximate Z of tissue?
7.4
70
What is the approximate Z of bone?
14
71
What is the approximate Z of prostheses?
20 - 25
72
What is the approximate density of lead?
11.34 g/cc
73
What is the approximate density of air?
0.0012 g/cc
74
What is the approximate density of lung?
0.33 g/cc
75
What is the approximate density of bone?
1.8 g/cc
76
What is the approximate density of tungsten?
19.25 g/cc
77
What is the approximate density of tissue?
1 g/cc
78
What is the equation for average energy at depth (z) for an electron beam?
Eo*(1-z/Rp)
79
What is the rule of thumb for dose calculation error due to HU value error?
10% error in HU value yields a 2% error in calculated dose
80
What is the average magnitude of tumor motion in lung? What is the absolute maximum?
1 cm average | 5 cm max
81
What is average out of field energy compared to in-field energy?
4-10% in field energy
82
What are typical doses for... kV planar: MV planar: kV CBCT: MV CBCT: 
What are typical doses for... kV planar: 0.5 - 1 mGy MV planar: 1-3 cGy kV CBCT: 1 cGy MV CBCT: 15-20 cGy
83
What are the following dmax's? (used in MD Anderson TLD check) ``` 6X 6XFFF 10X 10XFFF 15X ```
``` 6X - 1.4 cm 6XFFF - 1.4 cm 10X - 2.2 cm 10XFFF - 2.2 cm 15X - 2.7 cm ```
84
What are the following dmax's? (used in MD Anderson TLD check) ``` 6E 9E 12E 16E 20E ```
``` 6E - 1.3 cm 9E - 2.1 cm 12E - 2.9 cm 16E - 3.0 cm 20E - 2.0 cm ```

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