First Year Exam: Special Physics Flashcards
What is the α/β for the following…
Normal Tissue
Tumors
L Spine
Cauda
T Spine
C Spine
Prostate
Normal Tissue - 3
Tumors - 10
L Spine - 4
Cauda - 4
T Spine - 2
C Spine - 2
Prostate - 1.5
What is the equation for EQD2?
EQD2 = nd( (d+α/β) / (2+α/β) )
What is the equation for EQD2 using BED?
EQD2 = BED / (1+2/α/β)
For Chen, how are low, intermediate and high risk determined?
Low Risk - Cumulative Dmax < 95 Gy EQD2 and treatment interval > 2 years
Intermediate Risk - Only one of the following is true, the other is false: Cumulative Dmax > 95 Gy EQD2 or treatment interval < 2 years
High Risk - Cumulative Dmax > 95 Gy EQD2 AND treatment interval < 2 years
For Nieder, how are low, intermediate and high risk determined?
Low Risk - cumulative max dose < 150 Gy2 BED and interval > 6 months
Intermediate Risk - cumulative max dose > 150 Gy2 BED and < 170 Gy2 BED or interval or one course contributes > 102 Gy2 BED by itself
High Risk - cumulative max dose > 170 Gy2 BED.
What is the equation for BED?
BED = nd( 1 + d/α/β )
What does BED stand for?
Biological effective dose
What α/β corresponds to late responding tissues? Which correspond to early?
Lower α/β corresponds to late responding tissues
Higher α/β corresponds to early responding tissues
What are the four (arguably five) R’s of radiobiology?
Repair
Repopulation
Reoxygenation
Redistribution
Radiosensitivity (debated)
Would you use a higher or lower dose per fraction for late responding targets?
Higher dose per fraction
During which time period is the fetus most sensitive to radiation damage?
2 - 15 weeks post conception
What are associated abnormalities with irradiation during organogenesis?
Decreasing head size
Mental retardation
Congenital malformations
What is the largest component of scatter radiation at larger distances from the field?
Leakage
What is the largest component of scatter near field edges?
Scatter from within the patient
What three points would you put your dosimeters if you wanted to do in-vivo dosimetry for a pregnant patient?
Fundus, Symphysis pubis, umbilicus
What shielding is required for fetus in pregnant women?
Bridge over patient with 4-5 HVLs of lead
Above how much dose is there significant risk to the fetus?
10-50 cGy
Above what dose is there very high risk to fetus in any trimester?
And above what dose is there a 50% chance of killing fetus?
50 cGy
At 100 cGy there is a 50% probability of killing fetus
At about what distance from the field edge is the contribution of patient scatter and head leakage approximately equal?
20 cm (per TG 203)
What is the approximate % dose 10 cm away from field edge?
1% of central axis dose
What is the approximate % dose 30 cm from field edge?
0.2% of central axis dose
True or False
NCRP considers biological risk to fetus from photoneutrons to be negligible, therefore it is fine to treat with energies > 10 MV in pregnant patients
False
NCRP does consider the risk to be negligible, HOWEVER, it is still recommended to use 10 MV or less to avoid even the negligible contribution
What 5 planning recommendations would you give dosimetry for pregnant patients?
- Modify field size and angle to avoid irradiating direction of fetus
- Avoid wedges (decreases scatter)
- Use conventional 3D-CRT instead of IMRT (less leakage)
- Select a lower energy (easier to shield and less internal scatter at distance)
- Shield using bridge
What considerations/practices do you need to implement for planning hip prostheses?
- Make sure extended HU is turned on
- Use Acuros
- Call vendor to figure out what the prosthesis material is, and assign it to the plan
- Do not treat fields that enter or exit through the prostheses
- Manually correct streak artifacts