Background: Despite decreasing rates of subsequent contralateral breast cancer after diagnosis of unilateral primary breast cancer, the proportion of patients electing contralateral prophylactic mastectomy (CPM) is increasing. Our aim was to identify risk factors associated with the identification of occult malignancy (OM) or high-risk lesion (HRL) in CPM to facilitate patient counseling and operative planning. Methods: We identified patients undergoing CPM in addition to mastectomy for index breast cancer between October 2008 and June 2013. Patient and tumor factors were analyzed to identify associations with OM or HRL in CPM. Results: Among 740 CPM patients, an OM was identified in 4.1 % and an HRL was identified in 10.5 %. On multivariable analysis, factors associated with either occult finding included older age [odds ratio (OR) 1.37, per 10-year increase], invasive lobular index tumor histology (OR 2.60), progesterone receptor (PR)-positive index tumor (OR 1.79), and neoadjuvant therapy (OR 0.55). Overall, 244 patients (33 %) underwent BRCA testing, and 38 (16 %) had a deleterious mutation; 494 patients (67 %) had a preoperative breast MRI. Neither absence of a deleterious BRCA mutation nor a negative preoperative MRI decreased the likelihood of an occult finding in CPM. Conclusions: Although invasive cancer was identified infrequently in CPM specimens, the rate of HRL or OM in our study was 14.6 %. Older age and infiltrating lobular and PR-positive index breast cancers were associated with a higher risk of OM in CPM, while neoadjuvant therapy diminished the risk. BRCA testing and preoperative MRI were not associated with HRL or OM. This information is valuable for patient counseling and surgical planning.
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