Objective: Cancer is more likely to be diagnosed in the left breast than the right, but the reasons are undetermined. Left-sided predominance has not been evaluated for some demographic groups or by tumor location.
Methods: Laterality was analyzed among 419,935 incident unilateral breast cancers from 26 population-based cancer registries covering 40% of the US population. Logistic regression assessed the independent contribution of race, ethnicity, age, histology, stage, and location to laterality.
Results: Breast cancer was about 5% more likely to be diagnosed in the left breast than the right, a finding that was generally consistent across demographic groups and tumor types. Left-sided predominance was evident among both younger (<45 years) and older women, and among men with either in situ or invasive disease. Among women, tumors in the upper-outer quadrant, where one-third of cancers are located, occurred with equal frequency in the left and right breast, while those in the lower quadrants were about 10% more likely to occur in the left breast.
Conclusion: The observation that the left breast is at greater risk of cancer than the right may not apply to tumors arising in the upper-outer quadrant. The identification of physiologic, pathologic, or immunologic differences between the lower, but not upper, left and right breasts may assist in explaining breast cancer laterality.