Radiation Biophysics, 12 May 2003: Final Examination

  1. (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).
  2. (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.

  3. (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.

  4. (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.
  5. (20 points)  Describe two different mechanisms by which fetal damage might arise if the uterus is irradiated during the early stages of pregnancy.

  6. (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.

  7. (5 points)Define fixation of damage by oxygen and similar molecules.
  8. (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?
  9. (5 points)Describe the continuous slowing down approximation.
  10. (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?

  11. (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?

  12. (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?

  13. (5 points) Contrast the roles of the p53 gene product and the bcl-2 gene product in the context of apoptosis.

  14. (5 points)Why does the testis weight loss assay not follow simple exponential dose dependence, e.g. ln(W/W0) = -k * D?

  15. 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.