Organ-specific cancer risks following exposure to radiation can be explained by a shift of spontaneously arising cancers toward younger ages.

INTRODUCTION

Cancer risks of radiation are commonly made to evaluate an increase in mortality or incidence of cancers above background levels in subjects of the same age. However, the increased risk can also be evaluated by assuming a dose-dependent earlier onset expressed by a parallel shift of the mortality or incidence rate toward younger ages, which eventually results in life shortening.

METHODS

Organ-specific relative risks for cancer was estimated by assuming that exposure to radiation increases the risk due to tissue reactions which subsequently facilitate an earlier onset of naturally occurring cancers. In this case, the years of earlier onset X can be obtained by examining the equation showing that the mortality or incidence rate for all cancers at age 70 in the 1 Gy-exposed group equals the rate at age 70 + X in the control group. In the present study, assuming that the X can be applied to all organs, organ-specific relative risk (RR)/Gy values were calculated as the ratio of the mortality or incidence rate at age 70 + X vs. at age 70 in the control group.

RESULTS AND DISCUSSION: The RR/Gy values thus obtained agreed closely with the epidemiologically estimated RR/Gy in major organs (stomach, colon, lung, liver etc.) while no clear evidence for an increased risk has been observed in the Life Span Study of atomic bomb survivors for pancreas, gallbladder, and kidney although the age-related patterns for the incidence or mortality of the control subjects are similar to those for the major organs. Possible reasons for the discrepancy are discussed.