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Objective:
The reason for the slight excess of cancers in the left breast is unknown. This study describes characteristics of breast cancer laterality in a large national database that allows analysis of even very small effects.
Design:
Retrospective review of the 18 registry SEER database.
Patients:
Over a million cases of breast cancer from 1973 - 2009. Patients with simultaneous bilateral cancer or unknown laterality were excluded.
Results:
Over the 36 years of the study, the incidence of cancer in the left breast steadily decreased from 51.6% in the 1970’s to 50.8% in the 2000’s (p<0.001). There was no difference in the left sided predominance by race or ethnic group, but it steadily increased by patient age (50.5% for age < 50 vs. 51.3% for age >65, p<0.001). Interestingly, the left predominance was greater in males, 51.8%. There was no difference by tumor size or stage, but the predominance was greatest in high grade tumors (51.3% vs 50.1%, p<0.001) and in ER negative tumors (51.5% vs 50.6%, p<0.001). The predominance was greater for in situ than for invasive cancers (51.4% vs 50.9%, p<0.001), but it was similar for all major histologic types except for tubular carcinoma (p<0.001). Patient age, ER status, grade, and invasiveness were all significant in multivariable modeling.
Conclusions:
Although the clinical relevance of these findings is not yet clear, the data provide intriguing clues regarding the biology of breast cancer. The results do not support two previously proposed explanations that the effect is mediated by breast feeding practices or that the effect is merely due to the left breast being larger than the right.