The expected benefit of preventive mastectomy on breast cancer incidence and mortality in BRCA mutation carriers, by age at mastectomy

The age-specific annual risks of developing breast cancer were used to estimate the actuarial risk of developing breast cancer by age 80 for women with a BRCA1 or BRCA2 mutation. The probability of developing breast cancer before age 80 was then modified to include competing causes of death, including from ovarian cancer. The mortality rate from breast cancer after a diagnosis of breast cancer was set at 2% annually for the first 10 years and then 1% annually for years ten to twenty. The incidence rate and mortality rate from ovarian cancer were based on published literature. We assumed that preventive mastectomy was associated with complete protection against subsequent breast cancer. A series of simulations was conducted to evaluate the reduction in the probability of death (from all causes) until age 80, according to the age at mastectomy.

The actuarial risk of developing breast cancer until age 80 was estimated to be 70.8%. The actual risk (incorporating competing risks) was 64.0%. The probability of being alive at age 80 by having a mastectomy at age 25 increased by 8.7% (from 42.7 to 51.3%). The estimated benefit declined with age at mastectomy; for surgery done at age 50 the improvement in survival to age 80 was much more modest (2.8% at age 80, from 42.7 to 45.5%).