Purpose: We determined the risk of breast cancer and tumor characteristics among current postmenopausal hormone therapy users compared with nonusers, by duration of use.
Methods: From January 1996 to December 2000, data were collected prospectively on 374,465 postmenopausal women aged 50 to 79 years who underwent screening mammography. We calculated the relative risk (RR) of breast cancer (invasive or ductal carcinoma-in-situ) and type of breast cancer within 12 months of postmenopausal therapy use among current hormone users with a uterus (proxy for estrogen and progestin use) and without a uterus (proxy for estrogen use), compared with nonusers.
Results: Compared with nonusers, women using estrogen and progestin for >/= 5 years were at increased risk of breast tumors of stage 0 or I (RR, 1.51; 95% CI, 1.37 to 1.66), stage II or higher (RR, 1.46; 95% CI, 1.30 to 1.63), size </= 20 mm (RR, 1.59; 95% CI, 1.43 to 1.76), size greater than 20 mm (RR, 1.24; 95% CI, 1.09 to 1.42), grade 1 or 2 (RR, 1.60; 95% CI, 1.44 to 1.77), grade 3 or 4 (RR, 1.54; 95% CI, 1.37 to 1.73), and estrogen receptor-positive (RR, 1.72; 95% CI, 1.55 to 1.90). Estrogen-only users were slightly more likely to have estrogen receptor-positive breast cancer compared with nonusers (RR, 1.14; 95% CI, 1.06 to 1.23).
Conclusion: Use of estrogen and progestin postmenopausal hormone therapy for five years or more increased the likelihood of developing breast cancer, including both tumors with favorable prognostic features and tumors with unfavorable prognostic features.