Breast Size and Breast Cancer: There is a popular belief that small breasts are at lower risk of breast cancer. This theory has been used to explain why women with breast implants have a smaller risk of breast cancer. However, most studies have found no association between breast size and breast cancer risk. One study, however, did find an increase in the risk of breast cancer among lean women with larger breasts. In this study, more than 4,000 women were grouped according to their bra size before childbirth. Women who were lean (chest size less than 34 inches) and had larger breasts (size B, C or larger cups) were at higher risk of post menopausal breast cancer relative to women of the same chest size with an A or smaller cup size. Women with other chest sizes had no association between breast cup size and breast cancer risk. More studies are needed to confirm these results

Mammographic density: Numerous epidemiological studies have shown that breast density as measured on mammograms is a significant risk factor for breast cancer. The risk of breast cancer associated with the highest category of density has been estimated to be much greater than in the lowest density category. Mammographic density appears to be predictive for developing invasive cancer after DCIS (ductal carcinoma in situ). Increasing density is associated with increasing breast cancer risk in both premenopausal and postmenopausal women, with the effect persisting for ten years after mammography. Mammographic density has also been shown to be a risk factor for breast cancer in women with a family history of the disease. Mammographic density is probably important even in patients who are BRCA gene positive. In fact, mammographic density may actually have a substantial heritable component.