Further analysis of incidence of multiple myeloma among atomic-bomb survivors, 1950 to 1994

Multiple myeloma (MM) predominantly occurs in older persons and its diagnosis depends on pathological and laboratory findings. Epidemiological studies have provided inconsistent results on relationship between radiation exposure and MM. The Life Span Study (LSS) of atomic bomb (A-bomb) survivors has not shown a statistically significant radiation risk for MM. We studied previously reported MM cases diagnosed between 1950 and 1994 in the LSS and assessed diagnostic certainty based on histological and clinical features. Among 122 incident MM cases, 67 were classified as definite, 23, probable and 32, undetermined. The incidence rate increased with attained age and was highest in the oldest age group of over 70 years. For definite MM, Poisson regression dose response analysis showed an elevated, though not statistically significant, radiation risk with an estimated excess relative risk (ERR) per Gy of 0.44 (95%CI:<-0.02, 2.4), which was markedly higher than the previously reported ERR/Gy of -0.02 (95% CI: -0.24,0.75) that included cases with undetermined diagnostic certainty. The present case series included 24 persons with MM diagnosed after participating in M-protein screening offered to a clinical subset of the LSS members. Exclusion of screen-detected cases led to a higher radiation risk estimate, which was not statistically significant. A large number of young survivors exposed before age 20 years were younger than 70 years in the current follow-up period, but will reach ages with rapidly increasing MM rates. Future follow-up data with improved diagnostic certainty will improve understanding of long-term effects of radiation exposure on MM in this cohort.