BONE MASS AND THE RISK OF BREAST-CANCER AMONG POSTMENOPAUSAL WOMEN

Background Recent studies have shown a direct relation between serum e strogen levels assessed at a single point in time and the risk of brea st cancer, but no evidence links estrogen levels assessed repeatedly o ver an extended interval to the risk of breast cancer. Bone mass has b een proposed as a marker of cumulative exposure to estrogen in women. We therefore studied the association between bone mass and the inciden ce of breast cancer. Methods Between 1967 and 1970, 1373 women who wer e 47 to 80 years old and had no history of breast cancer underwent pos teroanterior hand radiography in the Framingham Study. We used radiogr ametry to measure the cortical width cif each woman's second metacarpa l. Participants were followed until the end of 1993. All incident case s of breast cancer were confirmed by pathological reports. We used a C ox proportional-hazards model to examine the relation of metacarpal bo ne mass to the risk of postmenopausal breast cancer. Results Postmenop ausal breast cancer developed in 91 subjects. Incidence rates per 1000 person-years increased from 2.0 among the women in the lowest age-spe cific quartile of metacarpal bone mass to 2.6, 2.71 and 7.0 among the women in the second, third, and highest quartiles, respectively. After adjustments for age and other potential confounding factors, the rate ratios for the risk of breast cancer were 1.0, 1.3, 1.3, and 3.5 from the lowest quartile to the highest (P for trend, <0.001). Conclusions Women in the highest quartile of bone mass are at higher risk for pos tmenopausal breast cancer than those in the lowest quartile. The mecha nisms underlying this relation are not understood, but cumulative expo sure to estrogen may play a part. (C) 1997, Massachusetts Medical Soci ety.