First Year Exam: Rules of Thumb and Values to Memorize Flashcards
What is the approximate pdd(10) for a 6MV beam?
65-67%
What is the approximate pdd(10) for a 6FFF beam?
63-64%
What is the approximate pdd(10) for a 10MV beam?
73-75%
What is the approximate pdd(10) for a 10FFF beam?
70-71%
What is the approximate pdd(10) for a 15MV beam?
76-77%
What is the approximate pdd(10) for a 18MV beam?
80%
What is the approximate dmax for a 6FFF beam?
1.4 - 1.5 cm
What is the approximate dmax for a 6MV beam?
1.4 - 1.5 cm
What is the approximate dmax for a 10FFF beam?
2.2 - 2.4 cm
What is the approximate dmax for a 10MV beam?
2.2 - 2.4 cm
What is the approximate dmax for a 15MV beam?
2.7 - 2.9 cm
What is the approximate dmax for a 18MV beam?
3.3 cm
What is the approximate dmax for a Co-60 beam?
0.5 cm
What is the approximate pdd(10) for a Co-60 beam?
55%
What is the attenuation rate dose fall-off rule of thumb for 6MV beams past dmax?
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
What is the attenuation rate dose fall-off rule of thumb for 10MV beams past dmax?
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
What is the attenuation rate dose fall-off rule of thumb for 18MV beams past dmax?
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
What is the average energy of a photon beam?
Max energy / 3
How is a change in flatness indicative of a change in energy?
A 10% change in flatness corresponds to a 1% change in energy
What is the average energy of an electron beam at patient surface?
Equal to Emax (since electron beams are close to monoenergetic)
If given E0, what is the approximate R50 in water?
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
If given E0, what is the approximate R80 in water?
R80(cm) = E0/2.9
If given E0, what is the approximate R90 in water?
R90(cm) = E0/3.2
If given E0, what is the approximate Rp in water?
Rp(cm) = E0/2
What is approximate bremmstrahlung tail for 6E, 12E and 20E respectively?
6E - 1%
12E - 2%
20E - 5%
What is approximate dmax of a 6E beam?
1.2 - 1.3 cm
What is approximate dmax of a 9E beam?
1.9 - 2.1 cm
What is approximate dmax of a 12E beam?
2.8 - 2.9 cm
What is approximate dmax of a 16E beam?
3.0 - 3.1 cm
What is approximate dmax of a 20E beam?
2.5 cm
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
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
What is the equivalent square field equation?
A/P = A/P or…
x^2/4x = AB/2(A+B)
Solve for x