Enigmatic sex disparities in cancer incidence

In this study we aimed to identify cancers where there is a consistent sex disparity, with the goal of identifying unexplained sex disparities that may offer promising opportunities for etiologic research. Age- and sex-specific cancer incidence data from Cancer Incidence in Five Continents, provided by the International Agency for Research on Cancer, were used to calculate incidence rate ratios for 35 cancer sites, comparing men to women, adjusting for attained age, gross domestic product (GDP), and geographical region. Genital cancers and breast cancer were excluded. The consistency of relative risks was examined by GDP and geographical region and, in a subset of longstanding cancer registers, by calendar year. For each cancer site, the sex disparity was broadly classified as plausibly explained by established environmental risk factors, partly explained, or unexplained. Cancer incidence was statistically significantly higher in men than women at 32 of 35 sites, with disparities >2-fold for 15 sites and >4-fold for 5 sites. For nearly all sites, the sex disparity was consistent across GDP groups and geographical regions. However, the incidence rate ratios varied considerably by age at diagnosis. The sex disparity for 13 cancer sites was considered to be entirely unexplained by known risk factors; these sites showed strikingly little variation in the incidence rate ratios over decades. Thus, the basis of many of the largest sex disparities in cancer incidence seems mostly unknown, highlighting the need for intensified research into its origins.