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In cohort studies, unbiased estimation of exposure-outcome associations requires selection of an appropriate reference group of unexposed individuals. We illustrate strategies for analyzing cohort data with multiple potential reference groups. We analyzed the association between radiation exposure and incidence of first primary solid cancer among 105,444 participants of the Life Span Study (Hiroshima and Nagasaki, Japan, 1958-2009). Potential reference groups included zero-dose survivors at different ground distances from the hypocenter (internal) and city residents who were not in either city at the time of the bombings (external). DS02R1 weighted absorbed colon doses were estimated by the DS02 dosimetry system. Piecewise constant hazard models estimated excess relative risks of first primary solid cancer. We focused on sex-averaged excess relative risks and the shape of the dose-response curve. A model with internal standardization provided a sex-averaged excess relative risk of 0.510, 95% confidence interval: (0.414, 0.612) per gray of weighted absorbed colon dose, as well as strong evidence of a curvilinear dose response among males (P = 0.008). Selection of not-in-city residents as the reference group resulted in a larger excess relative risk of 0.560, 95% confidence interval: (0.467, 0.657) per gray, and reduced evidence of a curvilinear dose response among males (P = 0.042). These differences were particularly apparent at weighted absorbed colon doses &lt; 1 gray. In cohort studies, selection of an appropriate reference group requires understanding of the nature of unmeasured confounding to which the results could be sensitive.

Selection of reference groups in the Life Span Study of atomic bomb survivors