Epidemiology of breast carcinoma II: factors related to the predominance of left-sided disease.

A study of the laterality of 980 patients with unilateral breast carcinoma revealed a left/right ratio of 1.26. Detailed analysis disclosed a significant association between left predominance and the following clinicopathologic features: menarch after age 13, age at diagnosis, parity especially among those between ages 40 and 54, and all histologic types except medullary tumors. When asynchronous bilateral carcinoma was documented, the disease first occurred more often in the left breast. Patients with simultaneous bilateral disease usually had a larger tumor on the left. Because differences in breast size were considered a contributing factor, relative breast volumes were computed from the mammograms of 174 healthy women; 55% were found to have a larger left breast. These data and other observations cited in the literature suggest that the asymmetry of breast carcinoma reflects differences in the sensitivity of the mammary glands to hormonal stimulation, resulting in unequal volumes of tissue at risk to develop carcinoma.