ANS National
 Meetings/
 Sessions

November 1994

Myron Pollycove

(Univ. of California, San Francisco)

6. Low-Level Radiation: Adaptive Responses and Decreased Carcinogenesis

                "Significant decreased cancer mortality associated with low- level radiation has been demonstrated with high statistical power in two epidemiologic studies: decreased lung cancer mortality associated with increased radon exposure of the U.S. population and decreased breast cancer mortality of women in Canada following multiple fluoroscopic examinations. The tendency to neglect or reject data that contradict the linear-no threshold theory of radiation carcinogenesis is supported by the knowledge that DNA damage with chromosome aberration and gene mutation may be produced by a single particle of ionizing radiation and so initiate a malignancy. Consideration is not given to biological defense mechanisms that could be enhanced by the stimulation of low-level increments of radiation.

                "Decreased lung carcinogenesis associated with inhalation of increased concentrations of radon has been demonstrated with excellent statistical power by the University of Pittsburg nationwide study. This study of 1729 counties containing nearly 90% of the U.S. population is based on 272 000 University of Pittsburgh measurements in 1217 counties combined with studies by 9 states and the U.S. Environmental Protection Agency. After deleting states with high retirement migration and counties with incomplete data, 1601 counties remain. Figure I shows plots of mean age-adjusted lung cancer mortality rates (m) for white males (a) and females (c) compared with mean radon levels (r) in homes of all counties within various ranges of r, along with the standard deviation of the mean, first and third quartiles, and the best linear fit to the data for individual counties. These mortality rates are corrected for smoking by use of Bureau of Census Population Surveys of smoking prevalence and BEIR IV risk estimates for smokers and nonsmokers and are shown together with the best linear fits, M = m/m₀ = A+ Br in Figs. 1band 1d.  BEIR IV theory lines are normalized lines with slope B increasing mortality at a rate of 7.3% / pCi l ^-1. After correction for variations in smoking frequency, there is a very strong tendency for lung cancer mortality to decrease with increasing mean radon level in homes, in sharp contrast to the increased mortality expected from the linear-no threshold theory. The discrepancy between theoretical and measured slopes is by 20 standard deviations. Rigorous statistical analysis of single and in combination 54 socioeconomic, 7 altitude and weather, and multiple geographic variables as possible confounding factors demonstrate no significant decrease in the discrepancy. The multiple independent requirements that a possible unknown confounding factor must meet make its existence highly improbable. A reasonable explanation is that stimulated biological mechanisms more than compensate for the radiation 'insult' and are protective against cancer in a low dose, low dose rate range.

                "Decreased carcinogenesis resulting from low-level radiation is well documented in the Canadian study of fluoroscoped women that includes 31 710 patients admitted to national sanitoriums between 1930 and 1952 and alive on January 1, 1950. The results relate deaths from breast cancer between 1950 and 1980 that occurred 10 or more years after first exposure to fluoroscopic radiation. The epidemiologic data shown in Table I and the associated fitted models were not presented graphically by the authors. The graph is shown in Fig. 2 together with an empirical polynomial function fitted to the data. The linear model (L) for 2-mGy exposures discards low-level data at 0.15 and 0.25 Gy, the data with the best confidence limits. Compared to the controls receiving 0 to 0.9 Gy, patients receiving 0.15 and 0.25 Gy demonstrate relative risks of 0.66 (p < 0.01) and 0.85 (p < 0.38), respectively. Nevertheless, the authors use linear models in Table II to predict the lifetime excess risk of death from breast cancer to be ~60 per million women after a single exposure to 1 cGy at the age of 30. Nine hundred excess deaths from breast cancer are predicted theoretically from the exposure of one million women to 0.15 Gy. In contradiction, their low-dose data predict with better than 99% of confidence limits that instead of causing 900 deaths, a dose of 0.15 Gy would prevent 10000 deaths in these million women.

                "The March, 1994 UNSCEAR report on 'Adaptive Responses to Radiation in Cells and Organisms' documents the multiple defense mechanisms at molecular, cellular, organic, and systemic levels involving enzymatic, hormonal, stress protein, and immunologic interactions that have been observed and confirmed by numerous investigators. These investigators demonstrate why the negative health effects observed at high levels of radiation that overwhelm these defense mechanisms cannot be extrapolated to low levels in which these stimulated defense mechanisms predominate with resultant decreased carcinogenesis."

1. B. L. COHEN, "Test of the Linear-No Threshold Theory of Radiation Carcinogenesis," Biological Effects of Low Level Exposures, E. J. CALABRESE, Ed., Lewis Publishers, Boca Raton, Florida (1994).

2. A. B. MILLER. G. R. HOWE, G. J. SHERMAN, J. P. LINDSAY, M. J. YAFFE, P. J. DINNER, H. A. RISCH, D. L. PRESTON, "Mortality from Breast Cancer After Irradiation During Fluoroscopic Examination in Patients Being Treated for Tuberculosis," New England J. Med,. 321, 1285 (1989).