1531 Background: Antagonizing estrogen with tamoxifen has set the precedent for preventing breast cancer. Aromatase inhibitors reduce contralateral breast cancer in the adjuvant setting more than tamoxifen. The goal of MAP.3 is to determine whether exemestane reduces invasive breast cancers in post-menopausal women at risk for the disease. Methods: NCIC CTG MAP.3 is a randomized placebo controlled double blind trial of exemestane versus placebo. Recruitment began in September 2004 in North America and Spain. Criteria for entry on MAP.3 include: a Gail score of 1.66%; previous benign breast disease including LCIS; age over 60. The target sample size for the trial is 4560. Baseline characteristics of women enrolled thus far on MAP.3 were reviewed to determine whether their projected risk of breast cancer is consistent with the assumptions made in designing the trial. Results: Baseline characteristics are available on the 1784 women enrolled to MAP.3 to date. Over 70% of participants are over 60 years old (mean 64; SD=8.2 years) and the majority of the women are Caucasian (85%). The average 5-year risk of breast cancer is 2.7% (median, 2.2%). However, a greater proportion of women over 60 (30%) have a substantially lower Gail score (<1.66) compared to women less than 60 years of age (8%). Few women in the MAP.3 cohort have been enrolled on a basis of a history of prior breast atypical hyperplasia or LCIS (8%), or DCIS treated with mastectomy (2%). Nearly half the MAP.3 participants (45%) have at least one first degree relative with malignant breast cancer, and 7.5% of these women have 2 or more first degree relatives with the disease. Conclusions: The risk profile of the women on MAP.3 will determine the event rate and therefore the time to trial unblinding and analysis. This initial description of the MAP.3 population enrolled thus far indicates a slightly lower baseline breast cancer risk in comparison to the risk profile of those enrolled in previous breast cancer prevention trials with tamoxifen or raloxifene (NSABP P1, IBIS-1 and STAR). Reasons for this difference are being explored. [Table: see text]
