Alleged Deadly toll of Chernobyl

["Ron L. Kathren" <rkathren@tricity.wsu.edu>]

Mr. Cohen --

You raised a very good question, and, in hopefully not too long winded
scientific style, I'd like to take a crack at responding.  The thirty or so
immediate deaths are indisputable, but the 15,000 claimed additional deaths
are based on assumptions with regard to radiation doses incurred by that
particular subpopulation and the risk of cancer induction per unit exposure
to ionizing radiation, as well as the size of the subpopulation.  Different
assumptions can produce vastly different estimates.

Although many like to claim that we know more about the effects of ionizing
radiation that any other insult, in actual fact the dose-response
relationship is not well known at the low dose end (and I will define low
dose here as < 100 mGy (10 rad) and, not surprising, differs for various end
points or specific cancers.  By multiplying an average dose estimated to
have been incurred in a given population by the estimated risk per unit dose
and the size of the population, one gets a sort of theoretical number of
deaths from cancer that will occur over a period of time, usually taken as
50 or 70 years.  For any specific radiogenic cancer, one can obtain from the
peer reviewed literature a rather wide choice of risk coefficients.  Thus, I
can manipulate the size of the estimated numbers of [theoretical] deaths
simply by appropriate selection of a risk coefficient from the scientific
literature as well as by the size of the population chosen or by the
estimated dose to that population.  Moreover, typically there is no
adjustment of this estimate for other causes of death, or the age at which
the exposure occurred Exposure of an elderly person theoretically increases
that individual's risk of dying of cancer, but if the exposure occurred at,
say, age 60, the real probability is likely zero as that person may die from
some other cause, likely heart disease or stroke, before the cancer has the
opportunity to manifest itself.

The whole question of low level radiation induction of cancer is a very
complex one, with many unknowns.  For safety and regulatory purposes, the
relationship is simplified by assuming what is known as the linear
nonthreshold response or LNT, which I personally believe to be a wise choice
as it likely errs on the side of safety. The LNT response assumes that the
risk of developing a fatal cancer is directly proportional to the dose, and
that any dose, no matter how small, carries with it a proportionate risk
(hence no threshold).  We know that the LNT does not hold for many cancer
endpoints, most notably for the dial painters, none of whom developed
osteogenic sarcoma at doses to the bone below 10 Gy (1000 rad).  Data for
other cancers is equivocal, and often confused by an apparent beneficial
effect (hormesis) at low doses.  Also, some people will never develop a
radiogenic cancer as they appear to have the right genetic makeup; we are
only now beginning to unlock these cytogenetic secrets.  (The same concept
applies to why not all heavy smokers develop lung cancer).

Hopefully the above has served to answer your question at least in part.
Rather than take a lot of time of  going back and forth with questions and
answers via e-mail and hopefully to provide you with a fuller discussion and
to avoid flame wars, I offer to continue this dialogue with you on an open,
honest, one-on-one basis.  So if you have further questions, feel free to
contact me directly by e-mail at:  rkathren@tricity.WSU.edu.  I will then
provide you with my telephone number and will be at your disposal should you
choose to call.

Ronald L. Kathren
Professor Emeritus
College of Pharmacy
Washington State University
rkathren@tricity.WSU.edu


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