Bone mineral density and risk of postmenopausal breast cancer.

To determine if higher bone mineral density (BMD) is a risk factor for breast cancer in women age 50 years and older. 37,860 women ≥ 50-year old with no previous breast cancer diagnosis had baseline BMD assessment between January 1999 and December 2007. Cox proportional hazards models were created for time to a new breast cancer as a function of lumbar spine or femoral neck BMD quartile (1st = lowest as reference) with adjustment for relevant covariates. A secondary analysis was performed to look for an association with estrogen receptor-positive (ER-positive) breast cancers. 794 invasive and in situ breast cancers (484 ER-positive) occurred with a median follow up of 5.4 years. Increased breast cancer risk was seen for the 3rd and 4th quartiles of lumbar spine BMD with hazard ratios (HRs) of 1.26 (95% CI, 1.01-1.58) and 1.45 (95% CI, 1.16-1.81), respectively and for the 3rd quartile of femoral neck BMD with a HR of 1.33 (95% CI, 1.07-1.64). A test for linear trend showed that lumbar spine BMD (P < 0.001) and femoral neck BMD (P = 0.04) were associated with increased risk. Higher lumbar spine BMD was also associated with increased risk of ER-positive breast cancer with HR of 1.45 (95% CI, 1.08-1.94), and 1.68 (95% CI, 1.24-2.27) for women in the 2nd and 4th quartiles, respectively. A test for linear trend showed lumbar spine BMD was associated with increasing risk of ER-positive breast cancer (P = 0.003). Increased ER-positive breast cancer risk was seen for the 3rd quartile of femoral neck BMD with a HR of 1.43 (95% CI, 1.08-1.89). Higher lumbar spine and femoral neck BMD are associated with higher risk of breast cancer in women ≥50-year old. Lumbar spine and femoral neck BMD are associated with increased risk of ER-positive breast cancer.