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Background: The etiology of male breast cancer is poorly understood, partly due to its relative rarity. Although tobacco and alcohol exposures are known carcinogens, their association with male breast cancer risk remains ill-defined. Methods: The Male Breast Cancer Pooling Project consortium provided 2,378 cases and 51,959 controls for analysis from 10 case-control and 10 cohort studies. Individual participant data were harmonized and pooled. Unconditional logistic regression was used to estimate study design-specific (case-control/cohort) ORs and 95% confidence intervals (CI), which were then combined using fixed-effects meta-analysis. Results: Cigarette smoking status, smoking pack-years, duration, intensity, and age at initiation were not associated with male breast cancer risk. Relations with cigar and pipe smoking, tobacco chewing, and snuff use were also null. Recent alcohol consumption and average grams of alcohol consumed per day were also not associated with risk; only one subanalysis of very high recent alcohol consumption (> 60 g/day) was tentatively associated with male breast cancer (ORunexposed referent = 1.29; 95% CI, 0.97-1.71; OR>0

The etiology of male breast cancer is poorly understood, partly because of its relative rarity. Although genetic factors are involved, less is known regarding the role of anthropometric and hormonally related risk factors. In the Male Breast Cancer Pooling Project, a consortium of 11 casecontrol and 10 cohort investigations involving 2405 case patients (n 1190 from casecontrol and n 1215 from cohort studies) and 52013 control subjects, individual participant data were harmonized and pooled. Unconditional logistic regression generated study designspecific (casecontrol/cohort) odds ratios (ORs) and 95% confidence intervals (CIs), with exposure estimates combined using fixed effects meta-analysis. All statistical tests were two-sided. Risk was statistically significantly associated with weight (highest/lowest tertile: OR 1.36; 95% CI 1.18 to 1.57), height (OR 1.18; 95% CI 1.01 to 1.38), and body mass index (BMI; OR 1.30; 95% CI 1.12 to 1.51), with evidence that recent rather than distant BMI was the strongest predictor. Klinefelter syndrome (OR 24.7; 95% CI 8.94 to 68.4) and gynecomastia (OR 9.78; 95% CI 7.52 to 12.7) were also statistically significantly associated with risk, relations that were independent of BMI. Diabetes also emerged as an independent risk factor (OR 1.19; 95% CI 1.04 to 1.37). There were also suggestive relations with cryptorchidism (OR 2.18; 95% CI 0.96 to 4.94) and orchitis (OR 1.43; 95% CI 1.02 to 1.99). Although age at onset of puberty and histories of infertility were unrelated to risk, never having had children was statistically significantly related (OR 1.29; 95% CI 1.01 to 1.66). Among individuals diagnosed at older ages, a history of fractures was statistically significantly related (OR 1.41; 95% CI 1.07 to 1.86). Consistent findings across casecontrol and cohort investigations, complemented by pooled analyses, indicated important roles for anthropometric and hormonal risk factors in the etiology of male breast cancer. Further investigation should focus on potential roles of endogenous hormones.