For prevention of high-risk breast cancer, previous options include selective estrogen receptor modulators such as tamoxifen that have been shown to reduce the risk of breast cancer by approximately 38%, but their own side effects (e.g., endometrial cancer, clotting abnormalities, etc.) and low prevention efficiency (95 NNT at 5 years, 56 NNT at 10 years) make it necessary to explore other preventive agents. drugs. NCIC CTG MAP.3 is a randomized, double-blind, placebo-controlled international multicenter phase III clinical trial evaluating exemestane for the prevention of breast cancer in postmenopausal women. Participating countries include the United States, Canada, France, and Spain. A total of 4560 postmenopausal women at high risk of breast cancer were enrolled from 2004 to 2010, and those enrolled had to have at least one risk factor: age greater than or equal to 60 years; Gail risk score greater than 1.66%; previous breast biopsy suggestive of ductal atypia or lobular atypia, or lobular carcinoma in situ, or previous ductal carcinoma in situ with total mastectomy. The results of the MAP.3 trial showed that exemestane not only reduced the incidence of invasive breast cancer by 65%, but also reduced the incidence of DCIS, ADH, ALH, and LCIS, and it is likely that these reductions in precancerous lesions will translate into a more pronounced reduction in invasive breast cancer over a longer follow-up period, suggesting that exemestane has a definite preventive effect on breast cancer development. In addition, this study did not identify serious safety concerns during the 3-year follow-up period, including an increased risk of osteoporosis, cardiovascular events, or other tumors.