Association between breast cancer laterality and tumor location, United States, 1994-1998

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.

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.

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.

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.