Radiation Protection and Clinical Radiobiology
Question #1:
Given that tissue tolerance is 60 Gy at 2 Gy/fraction and 40 Gy at 4 Gy/fraction. What is
its α/β ratio?
Question #2:
A breast patient is originally planned to receive a total dose of 4256cGy in 16 daily
fractions. However, after 6 fractions into the treatment, the Oncologist wants to deliver
the rest of the treatment in 2Gy fractions. What will be the new schedule that will
maintain the same overall biological dose to the tumor?
Question #3:
A patient was to receive radiation treatment of 2000cGy in 10 daily fractions. Due to
documentation error, 600cGy was delivered in the first 6 daily treatments. What will be
the total dose to be delivered in the next 7 treatment days (in equal fractions) that will
give the same biological dose to the tumor?
Question #4:
A pregnant patient underwent a PET-CT examination. Assuming a typical effective dose
from this type of exam to be 15 mSv estimate:
a) Effective dose to the foetus
b) Risk of childhood cancer
c) Risk of severe mental challenge
Are any of the radiation related risks significant enough to recommend medical abortion?
Question #5:
A middle-aged woman underwent 12 screening mammograms. Each mammogram study
consists of 2 views for each breast and delivers 3.7 mGy to both breast.
a) What is the effective (whole body) dose in mSv that the patient received from
each mammogram set? Hint: use the ICRP 103 tissue weighting factor for breasts.
H =
b) What is her lifetime risk of radiation induced fatal cancer stemming from the 12
mammographic studies?
Question #6:
Consider radiation treatment room design, as described in the lecture:
Calculate the annual dose equivalent at point B5 (behind the primary barrier) using the
formula:
W ⋅10
3
⋅ U ⋅ T ⋅10
− n
p
d
2
where W is the annual primary workload in Sv/yr
U is the use factor,
T is the occupancy factor, d is the distance from target to the calculation point located 0.3m beyond
the shielding barrier,
and n is the total number of tenth value layers (TVLs);
Use the following assumptions:
W = A maximum of 40 patients are treated over 10 hours on each of 250 days per year
to an unattenuated isocentre dose of 3 Gy. The annual workload, allowing additional 8%
for calibrations, maintenance and research can be estimated at 35,000 Sv/yr. Conservative
assumption all workload is at the highest photon energy of 15 MV (not 10 MV as in the
class).
U = use factor is 0.25
T = occupancy factor is 0.5
d = 7.8 m.
n = For 256 cm of concrete (primary) for 15 MV x-rays
Use the graph below to estimate the tenth value layer (TVL) and n at 15 MV:
From IPSM Report No. 46
Question #7:
Alternative radiation treatment modalities. Medulloblastoma is the most common
malignant brain tumor in children. Consider a pediatric patient to be treated with craniospinal
irradiation to the entire CNS. Available radiation modalities include:
a) Megavoltage x-rays
b) Neutrons, and
c) Protons
Which modality would you recommend and why?
