A milligram of prevention.

Earlier this year, the USPSTF gave a grade B recommendation for primary chemoprevention of breast cancer in women at “high risk.” In the IBIS-II trial, nearly 4000 postmenopausal women at high risk of breast cancer were randomized to 1 mg anastrazole daily versus placebo for 5 years. The targeted populations were women ages 40-44 with an estimated risk of breast cancer 4x that of the general population, women 45-60 with 2x the risk, and women 60-70 with 1.5x the risk. With a median follow-up of over 10 years, the incidence of invasive and non-invasive breast cancer was reduced by 49%, or from 8.5% → 4.4%. During the initial 5 years on study, that reduction was 67%. While the reduction was 37% after 5 years (i.e. after stopping the pill), it remained significant. Roughly 80% of incident cancers were invasive, and 75% were ER(+). The reduction in ER(-) tumors, although numerically notable at 27%, was not statistically different. TBL: Anastrozole taken as breast cancer chemoprevention for 5 years significantly reduces long term risk of breast cancer development in post-menopausal women at baseline increased risk. | Cuzik, Lancet 2019