Re: LNT theories

["Otto G. Raabe" <ograabe@ucdavis.edu>]

>In a message dated 12/7/99 12:14:22 PM Central Standard Time, 
>ograabe@ucdavis.edu writes:
>
>Cohen has correctly pointed out the ecological fallacy is negated if the
> linear no-threshold theory is correct for radiation risk since in that
> situation the observed risk to a group of people would not depend on how
> the dose was distributed among the group members. Hence, Cohen's studies
> are a valid test of the LNT theory and cannot be discounted by the
> ecological fallacy.
>
>-------------------------
>Prof. Raabe,
>
>Which LNT theory do you feel Dr. Cohen is testing?
>
>R. William Field
**********************************************************************
December 8, 1999
Davis, CA

The issue of the shape of the response curve for ionizing radiation
carcinogenesis has focused on the common use of simplistic linear
no-threshold (LNT) models that assume that risks at lower cumulative doses
are proportional to risks at cumulative higher doses. Usually, only the
risks at higher doses can be quantified with reasonable precision. Further,
since these risks are assumed to be based on simple stochastic phenomena,
the unequal distribution of radiation absorbed dose among members of a
group of exposed persons does not alter the overall risk.
 
In the recent radon models of indoor exposure to radon decay products,
actual radiation doses are not used. Rather, a surrogate for potential
cumulative dose is used, namely the estimated average exposure presented as
average radon air concentration. The actual radiation dose of interest is
the alpha radiation dose to the bronchial epithelium from radon and its
short-lived decay products. Since most of this dose comes from the airborne
radon decay products, the state of equilibrium is important, as well as
several other aerosol factors. Although the radon concentration is not the
dose, but it may be expected to be proportional to the dose on the average
if the relevant dosimetric conditions are similarly distributed among
exposed persons. 

Recognizing these limitations, Cohen has tested the linear model that
assumes that the occurrence of lung cancer as a result of exposure to radon
in homes is proportional to the average radon air concentration in these
homes. He has shown that it is not. His results cannot be discounted as
caused by the ecological fallacy since, as we can see inductively: if the
LNT model that he is testing is actually correct, then the distribution of
the radon levels among the exposed persons at a given average concentration
would not alter the overall occurrence of lung cancer. On the other hand,
if the LNT model that he is testing is wrong (as he has shown), then the
shape of the dose response curve that he has observed is not definitive and
it becomes apparent that we need to known dose to each person to quantify
the risk and the shape of the curve. Hence, the downward slope of his dose
response observation may be an artifact.

This is my interpretation of Cohen's test of LNT. I hope Dr. Cohen will
comment.

Otto

		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
              Institute of Toxicology & Environmental Health (ITEH)
		   (Street address: Building 3792, Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 530-752-7754  FAX: 530-758-6140
		E-mail ograabe@ucdavis.edu
              *****************************************************
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