Cancer

Background:Increasingly, breast cancer patients undergo bilateral mastectomy (BLM). The magnitude of benefit is unknown.Methods:We used SEER data on all women diagnosed in California from 1998-2015 with stage 0-III unilateral breast cancer and treated with BLM versus breast conserving therapy including surgery and radiation (BCT) or unilateral mastectomy (ULM). We measured relative risks of second contralateral breast cancer (CBC) and breast cancer death using Fine and Gray multivariable regression modeling adjusted for the competing risk of death and death from another cause, respectively, and potential confounding factors. We measured absolute excess risk (AERs) of CBC as the observed minus expected number of breast cancers in the general population, divided by 10,000 person-years at risk.Results:Among 245,418 patients with median follow-up of 6.7 years, 7,784 (3.2%) developed CBC. Relative risks were lower after BLM (hazard ratio (HR) 0.10, 95% confidence interval 0.07-0.14) and higher after ULM (HR 1.07, 1.02-1.13) versus BCT. AERs were higher after BCT and ULM (5.0 and 13.6 more cases, respectively) than BLM (28.6 fewer cases). BLM reduced risk more for older women (38.0 fewer cases for age 6550, versus 17.9 among age