Radiation Biophysics, 12 May 2003: Final Examination
- (20 points)
(a) In the figure below, C is the
survival curve for a cell line irradiated under anoxic conditions.
Two other conditions are tested: low but nonzero P[O2],
and atmospheric oxygen pressure. Which correspond to these two
conditions? Why?
(b) Given the above, would you expect that the low-but-nonzero
P[O2] mentioned above be closer to 0.1 atm or 0.01 atm?
Why? (hint: atmospheric P[O2] = 0.19 atm, roughly).
- (20 points)
The results of a ficticious lung cancer study are as follows.
Three populations were identified: nonsmokers, light smokers,
and heavy smokers. Radiation exposure was monitored for the
three groups and it was found that the cancer incidence in the
three groups showed stochastic sensitivity to dose, behaving
according to linear-quadratic statistics. The linear-quadratic
coefficients associated with Alpen's eqn. (12.2) were:
population | In |
1, Sv-1 |
2, Sv-2 |
nonsmokers | 2*10-4 |
1*10-4 |
0.5*10-4 |
light smokers | 10*10-4 |
10*10-4 |
1.33*10-4 |
heavy smokers | 20*10-4 |
20*10-4 |
2*10-4 |
(a) Do these data fit a relative risk model or an absolute risk model
for radiation carcinogenesis? Explain.
(b) Describe the ways in which these results are consistent with the
the initiator-promotor model of carcinogenesis.

- (20 points)
Explain why a plot of relative biological effectiveness versus linear energy
transfer for a constant dose shows
RBE increasing as a function of increasing LET, up to a point but no further.
Include in your explanation the reason that d(RBE)/d(LET) < 0 at
very high values of LET.
- (20 point)
Patients afflicted with ataxia telangiectasia (AT) are at high risk
for numerous health problems arising from their tissues' inability to
repair certain kinds of DNA damage. Homozygotes for AT rarely survive
to adulthood. Heterozygotes rarely show symptoms while young but are
at substantially incrased risk for contracting several forms of cancer
as adults. Explain.
- (20 points) Describe two different mechanisms
by which fetal damage might arise if the uterus is irradiated during
the early stages of pregnancy.
- (20 points) Macromolecular crystallography consists of
irradiating small crystals of highly hydrated proteins with X-rays and
then measuring the diffraction patterns that emanate from the crystals.
Protein crystals usually float, just barely.
Suppose a cubical protein crystal of dimension 0.1 mm is irradiated
with 1011 X-ray photons/second,
using 10 KeV X-rays. Assume that 0.01% of the incident energy
is actually absorbed in the protein crystal.
Calculate how many Gy of dose is received by
the crystal in a typical 1-hour experiment.
- (5 points)Define fixation of damage by oxygen and similar molecules.
- (5 points)In an Elkind-Sutton-style experiment, does the slope
of the linear portion of the log(survival) curve depend on the time delay
between the initial dose and the final dose?
- (5 points)Describe the continuous slowing down approximation.
- (5 points)A chromosome is damaged by radiation in such a way as
to produce a product that has two centromeres. What is this product called?
- (5 points) If a human inhales radionuclides adsorbed onto particles
of diameter greater than 10 micrometers, is he or she more likely to develop
tumors in the throat or in the deep lung? Why?
- (5 points) A patient receives a 0.5 Gy whole-body dose of 10 MeV
neutrons as part of a therapeutic regimen. What is the equivalent dose in Sv?
- (5 points) Contrast the roles of the p53 gene product and the bcl-2
gene product in the context of apoptosis.
- (5 points)Why does the testis weight loss assay not follow simple
exponential dose dependence, e.g.
ln(W/W0) = -k * D?
- Extra credit: (10 points)
(a) Show that dose rate is equal to power absorbed per unit
mass of absorber.
(b) Dose rate is given in units of dose per unit time, e.g.
Gy s-1.
Express this unit in basic MKS (meter-kilogram-second) units.
You may treat the coulomb as a basic unit.