Radiobiology > Post Test Answers


Post Test Answers

1. A victim of a nuclear power plant meltdown receives a dose of 5000 rads. Which of the following represents the equivalent in SI units?

    Answer:  50 Gy or 5000 cGy

Note that the Gray is the SI unit for absorbed dose; it is very important to know the conversion from rads to Gy.   1 rad = 0.01 Gy = 1 cGy = 10 mGy 

Brateman, L. The AAPM/RSNA Physics Tutorial for Residents: Radiation Safety Considerations for Diagnostic Radiology Personnel.  Radiographics.
1999; 19:1037-1055.


2. It is incumbent on the radiologist to use medical x-rays judiciously and avoiding unnecessary radiation...

  • Because of the increasing use of medical and dental x-rays for screening and diagnosis
  • Since diagnostic x-rays are the greatest single source of exposure to man-made radiation of the human population today. 
  • Since there is no dose of radiation that can be considered “safe” or "harmless."

    Answer:  All are true

The Board on Radiation Effects Research of the National Academy of Sciences. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 (2006).http://www.nap.edu/books/030909156X/html, Public Summary.

3. A Petri dish of mammalian cells in an oxygenated environment is irradiated and the surviving fraction is calculated to be 1%. The dish receives an additional 150 rad, what is the expected surviving fraction?

     Answer:  0.0037

This question requires you recall the D0 for oxygenated mammalian cells, which is 1.5 Gy (150 rad). Thus, 37% cell survival is expected after 1.5 Gy.  (0.01*0.37=0.0037)

Hall E.J. Radiobiology for the Radiologist. 5th edition. Philadelphia, PA: Lippincott Williams & Wilkins, 2000: 528.

4. T/F: In the presence of dissolved O2 the number of free radicals is increased.

    Answer: True. 

Therefore DNA damage may be greater in well-oxygentated tissues. Oxygen is a radiosensitizer.

Hall E.J. Radiobiology for the Radiologist. 5th edition. Philadelphia, PA: Lippincott Williams & Wilkins, 2000: 91.

5. T/F : The shoulder of a cell survival curve for x-rays, when considered for low doses delivered in a fractionated manner, leads to exaggerated RBE as compared with higher LET radiation having log-linear killing.

    Answer: True. 

This somewhat convoluted statement expresses a simple concept: for each low dose fraction, the shoulder of an x-ray cell survival curve repeats. Thus, as the absorbed dose is considered in terms of summed fractions, the cell population will decrease overall but at a shallower slope than that described by the single dose curve. Contrast this with the log-linear killing of higher LET radiations (note that linear implies no shoulder) which have identical population effect regardless of whether the total absorbed dose is singular or fractionated.

Hall E.J. Radiobiology for the Radiologist. 5th edition. Philadelphia, PA: Lippincott Williams & Wilkins, 2000: 32-50.

6. T/F Cell killing of ionizing radiation is enhanced in the presence of oxygen, and there is greater enhancement for lower LET radiations.

    Answer: True

Pizzarello D.J. and R.L. Witcofski. Basic Radiation Biology. Philadelphia, PA: Lea & Febiger, 1970.

7. Which of the following symptoms would occur ONLY if a total-body acute exposure exceeded 8,000 rad (80 Gy)?

    Answer:  Hallucinations and impaired vision

The CNS effects seen in the Cerebrovascular Syndrome require doses approaching 100 Gy. All the other effects may be seen at doses less than 20 Gy. Note that hair loss occurs after about 3 Gy.

Hall E.J. Radiobiology for the Radiologist. 5th edition. Philadelphia, PA: Lippincott Williams & Wilkins, 2000: 124-135.  


8. Which of the following statements is FALSE?

    Answer:  The Doubling Dose for humans is estimated to be 0.5 Sv. 

The Doubling Dose for humans is estimated to be 1-2 Sv. A population exposed to this dose would expect to experience a doubling of its spontaneous mutation rate.

The Board on Radiation Effects Research of the National Academy of Sciences. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2 (2006).http://www.nap.edu/books/030909156X/html, Public Summary: Estimating Risks to Children of Parents Exposed to Ionizing Radiation.


9. Which is FALSE concerning stochastic processes:

    Answer:  Often have a threshold dose

Review the definition for stochastic...

International Commision on Radiological Protection. Radiation and your patient: A guide for medical practitioners (ICRP Supporting Guidance 2). 2001: 13-17.  http://www.icrp.org/educational_area.asp

10. For which of the following diagnostic examinations performed on an unknowingly pregnant patient should therapeutic abortion be considered because of putative risk from radiation?

    Answer:  None of the above

It is highly unlikely that any combination of diagnostic procedures would produce a radiation exposure justifying such a recommendation (i.e., in excess of 0.1 Gy or 100 mGy). And even if the fetus were exposed to 0.1 Gy (10 rads) consider that 100 normal pregnancies would be terminated to prevent one excess malformed fetus from completing development.

UNSCEAR 2000 Report to the General Assembly, with scientific annexes Vol. II: Sources and Effects of Ionizing Radiation. Annex G: Biological Effects at Low Radiation Doses. United Nations Scientific Committee on the Effects of Atomic Radiation. Updated July 25, 2006.  Accessed October 22, 2006.

11. The highest risk of structural anomalies probably occurs when human embryos or fetuses are exposed to ionizing radiation during which part of the gestational period?
 
    Answer: 18-45 days post-conception

Review in utero exposure... 

Valentin J., ed. International Commission on Radiological Protection, Publication 90: Biological Effects after Prenatal Irradiation (Embryo and Fetus). 2003; 33:1-206.


12. A child was exposed to 4 rads (40 mGy) of radiation during the 9th week of gestation – his radiation exposure may put him at greatest risk for which of the following: 

    Answer:  Acute Lymphocytic Leukemia at age 4 years old

For epidemiologic studies other than the Oxford Survey of Childhood Cancers, leukemias demonstrate the greatest increased relative risk following in utero radiation exposure. Lymphocytic cancers tend to present at 4-5 years old. (Myeloid cancers may present 5-9 years old.) At nine weeks GA, the patient has completed most major organogenesis, with the exception of neural tissues, and four rads is unlikely to produce any of the other dramatically abnormal effects listed.

Valentin J., ed. International Commission on Radiological Protection, Publication 90: Biological Effects after Prenatal Irradiation (Embryo and Fetus). 2003; 33:1-206.

     



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