Synchronous bilateral breast cancers frequently share the same estrogen receptor (ER) status, yet may differ in other histopathologic features. We sought to examine concordance rates of Oncotype DX recurrence score (RS) testing in women with synchronous bilateral ER-positive breast cancer.

Institutional databases were reviewed to identify patients with synchronous (within 6 months) bilateral primary invasive breast cancer and multiple RSs. RSs were stratified by risk group (RS < 18, low; RS 18-30, intermediate; RS ≥ 31, high) and considered discordant if they reflected different risk groups.

From 2005-2014, a total of 115 patients presented with synchronous bilateral invasive breast cancer; 43 (37 %) had two RSs available. Median patient age was 60 years (42-84), median tumor size was 1.2 cm (0.5-3.7), and all cases were HER2 negative and node negative. Of 86 RSs, 63 (73 %) were low risk, 20 (23 %) were intermediate risk, and 3 (3 %) were high risk. RSs were concordant in 29 (67 %) patients. Patients with concordant RSs were older (62 years vs. 56 years) and had median levels of progesterone receptor (PR) expression that were higher and more similar-80 and 85 % for bilateral cancers, respectively, compared with 55 and 75 % for bilateral cancers in discordant cases. Discordant RS led to a treatment change in 8/14 (57 %) cases.

Among women with synchronous bilateral ER-positive HER2-negative breast cancer, RSs were concordant in 67 % of cases. Concordance rates may be higher in older women or among those with comparable levels of PR expression. These data suggest that testing of both tumors should be considered in patients who are candidates for adjuvant chemotherapy.