Cancer Incidence in Atomic Bomb Survivors. Part II: Solid Tumors, 1958-1987

This report presents, for the first time, comprehensive data on the incidence of solid cancer and risk estimates for A-bomb survivors in the extended Life Span Study (LSS-E85) cohort. Among 79,972 individuals, 8613 first primary solid cancers were diagnosed between 1958 and 1987. As part of the standard registration process of the Hiroshima and Nagasaki tumor registries, cancer cases occurring among members of the LSS-E85 cohort were identified using a computer linkage system supplemented by manual searches. Special efforts were made to ensure complete case ascertainment, data quality and data consistency in the two cities. For all sites combined, 75% of the cancers were verified histologically, 6% were diagnosed by direct observation, 8% were based on a clinical diagnosis, and 12.6% were ascertained by death certificate only. A standard set of analyses was carried out for each of the organs and organ systems considered. Depending on the cancer site, Dosimetry System 1986 (DS86) organ or kerma doses were used for computing risk estimates. Analyses were based on a general excess relative risk model (the background rate times one plus the excess relative risk). Analyses carried out for each site involved fitting the background model with no dose effect, a linear dose-response model with no effect modifiers, a linear-quadratic dose-response model with no effect modifiers, and a series of linear dose-response models that included each of the covariates (sex, age at exposure, time since exposure, attained age and city) individually as effect modifiers. Because the tumor registries ascertain cancers in the registry catchment areas only, an adjustment was made for the effects of migration. In agreement with prior LSS findings, a statistically significant excess risk for all solid cancers was demonstrated [excess relative risk at 1 Sv $({\rm ERR}_{1\ {\rm Sv}})=0.63$; excess absolute risk (EAR) per 104 person-year sievert (PY Sv) = 29.7]. For cancers of the stomach (${\rm ERR}_{1\ {\rm Sv}}=0.32$), colon (${\rm ERR}_{1\ {\rm Sv}}=0.72$), lung (${\rm ERR}_{1\ {\rm Sv}}=0.95$), breast (${\rm ERR}_{1\ {\rm Sv}}=1.59$), ovary (${\rm ERR}_{1\ {\rm Sv}}=0.99$), urinary bladder (${\rm ERR}_{1\ {\rm Sv}}=1.02$) and thyroid (${\rm ERR}_{1\ {\rm Sv}}=1.15$), significant radiation associations were observed. There was some indication of an increase in tumors of the neural tissue (excluding the brain) among persons exposed to the bombs before age 20. For the first time, radiation has been associated with liver (${\rm ERR}_{1\ {\rm Sv}}=0.49$) and nonmelanoma skin (${\rm ERR}_{1\ {\rm Sv}}=1.0$) cancer incidence in the LSS cohort. The present analysis also strengthened earlier findings, based on a smaller number of cases, of an effect of A-bomb radiation on salivary gland cancer. There was no significant radiation effect for cancers of the oral cavity and pharynx as a group, esophagus, rectum, gallbladder, pancreas, larynx, uterine cervix, uterine corpus, prostate, kidney and renal pelvis. Analyses of solid tumors individually and in combination revealed no appreciable differences between Hiroshima and Nagasaki (P > 0.5). The combined solid tumor analysis demonstrated a twofold greater relative risk for females than males and a trend for a decreasing relative risk with increasing age at exposure (P < 0.001). Females had a higher relative risk of cancers of the lung, total respiratory system and urinary system than males. The excess relative risk decreased with increasing age at exposure for combined digestive, stomach, nonmelanoma skin, breast and thyroid cancers. For solid cancers combined, the excess cancer risk increased with increasing attained age and was proportional to the background incidence rate. Unadjusted for age at exposure, the excess relative risk for most sites tended to decrease with increasing attained age. For some cancers (colon, breast, central nervous system and kidney) models that allowed the excess relative risk to vary with attained age fit at least as well as models that included age-at-exposure effects. For all solid tumors, excess cancers increased with time since exposure, based on an absolute excess risk model. Averaged over all ages at exposure, the relative risk decreased with time since exposure. Examination of temporal patterns by age-at-exposure groups suggested that the excess relative risk decreased with time for the younger age-at-exposure groups and remained virtually constant for the older cohorts. The LSS has served as one of the major sources of data used for cancer risk estimation. Previous studies focused primarily on the association between cancer mortality and radiation exposure. Although these mortality studies are extremely valuable, the accuracy of cancer diagnoses is limited, and death certificates do not provide adequate information on cancers with relatively high survival rates. Although incidence data also have their limitations (e.g., incomplete case ascertainment and partial reliance on death certificate diagnoses), they can provide more complete data on cancers with good survival, on histological type and on time from exposure to cancer onset. Thus future analyses of atomic bomb survivors should focus on both cancer mortality and incidence.