Survival analyses of atomic bomb survivors in Hiroshima Prefecture, Japan, 1968-1982--cancer mortality risk among early entrants.

We examined the mortality risk due to all causes of death and due to malignant neoplasms during 1968-82 among 204,209 atomic bomb survivors, including 49,215 early entrants. We used data compiled by the Research Institute for Radiation Biology and Medicine at Hiroshima University, which conducts mortality surveillance of these survivors in Hiroshima Prefecture, Japan. The purposes of this study were to investigate whether there was any relationship between exposure status and mortality risk among survivors, not altered by adjustment for confounding factors, and whether there were any differences among early entrants to the region within 2 km of the hypocenter after the bombing in mortality risk associated with date of entry and duration of stay. The mortality risk in directly exposed survivors decreased with distance from the hypocenter, even after adjustment for confounding factors. Entrants who entered the region on the day of the bombing had a significantly higher risk of mortality due to malignant neoplasm than those who entered thereafter, even after adjustment for the length of stay. The same results were obtained throughout the study period.