Laterality of Breast Cancer Incidence
The observation that breast cancer is more common in the left than the right breast has been of interest to the medical community for at least 50 years. However, the percentages you cite seem rather high. Data from a pooled analysis of 18 US studies was reported in 1959 and showed a four percent excess in left breast cancers. Several contemporary European studies found somewhat higher rates for left sided breast cancers in the range of 1.05 to 1.2 times the incidence on the right.
A satisfactory explanation for the excess incidence of left breast cancers has not yet been elucidated. However, it has been well established that, on average, the left breast is slightly larger than the right. This lends itself to the logic that more breast tissue is present to be at risk for the development of a cancer. A correlation between bra cup size (as a measure of mammary gland size) and cancer risk, particularly for postmenopausal women, was demonstrated by Hsieh et al. Handedness has also been incriminated as a cause for increased left sided breast cancers. However, two large (>1700 women) series from UCSF and Harvard show no definite link between either left or right handedness and breast cancer risk.
One of the largest and most recent series to be examined regarding breast cancer laterality was by Anders et al. from the Harvard school of public health. They showed that of 80,784 cases of invasive cancer and 3,835 cases of pre-invasive cancer left sided disease was more common only after reaching the age of 45. For women younger than 45, right handedness, never having been pregnant, and late age for beginning to have menstrual periods was associated with a somewhat higher incidence of right breast cancer. The finding of increased overall incidence of left breast cancer also extended to males. The authors can not conclude a reason for laterality of breast cancer, but do subscribe to the explanation of more breast tissue mass on the left as the likely explanation.
We are unaware of any ongoing studies to explain the modest imbalance of left breast cancer. It is difficult to ascribe it to one factor given the host of genetic, hormonal and environmental factors that may play a role in the pathogenesis of breast cancer. A possible contributing explanation seems the larger average breast size on the left.
The following are some references for more information on breast cancer laterality: