Abstract P2-08-22: Clinical implication of HER2/neu status in hormone receptor positive pure mucinous breast cancer

Abstract Background: Mucinous carcinoma of the breast is a rare type of breast cancer with favorable outcome compared with other types of breast cancer. The current guideline does not recommend chemotherapy/anti-HER2 therapy for mucinous breast cancer with hormone receptor-positive subtype regardless of HER2/neu status. In this study, we evaluated the survival of pure mucinous breast cancer according to tumor stage and subtype. Methods: Between 1989 and 2014, in Asan Medical Center, Korea, total 473 pure mucinous carcinomas (stage I-III) undergone curative surgery were reviewed retrospectively. 5yr disease-free and overall survival were analyzed according to size, lymph node metastasis, hormone receptor/HER2 status and given therapy. Result: Total of 473 patients with pure mucinous breast cancer were analyzed and median follow-up duration was 78.00 months. 439 patients were hormone receptor-positive, 374 were node negative, 55 were HER2/neu positive. Among 374 patients with hormone receptor-positive and node-negative, tumor size was <1cm in 46 patients, 1-2.9cm in 259 patients, ≥3cm in 69 patients. In HR-positive/Node-positive BCs, 90.8%(59/65)were given chemotherapy and 35.3%(6/17) were also given trastuzumab. Sixteen patients given trastuzumab were only included in the analysis to assess the benefit of trastuzumab among HER2 positive BCs. Overall, 5-year disease-free survival (DFS) rate was 94.1% and the 5-year overall survival (OS) rate was 95.9%. Using Cox regression analysis, lymph node metastasis was the only significant prognostic factor for both DFS (HR4.0, 95%CI:1.8-9.0, p=0.001) and OS (HR3.5, 95%CI:1.3-8.9, p=0.008). Among HR-positive/node-negative with tumor size ≥3cm, HER2/neu positivity was only significantly associated with 5yr-DFS (71.4% in HER2/neu+ vs. 96.4% in HER2/neu-, HR9.5, 95%CI:1.3-67.5, p=0.024). This observation was consistently combining both 'HR-positive/node-negative/>3cm' and 'HR-positive/node positive' BCs (N=127) that HER2 positive tumors showed worse survival (HR 3.7, 95%CI:1.2-10.8, p=0.015). Intriguingly, within this subgroup of HR-positive/node-negative/>3cm' and 'HR-positive/node positive' BCs, among HER2 positive tumors, while 5yr-DFS was 63.7% in patients who didn't receive trastuzumab, 100% were disease free in patients who were given trastuzumab. Conclusions: Overall, nodal status was the most significant prognostic factor for pure mucinous breast cancer. In hormone receptor-positive, lymph node negative mucinous breast cancer with tumor of ≥3cm, HER2 positive BCs showed worse survival, suggesting a potential role of anti-HER2 strategy in this subgroup. Citation Format: Gwark S-C, Lee JW, Lee SB, Sohn G, Kim J, Chung IY, Ko BS, Kim HJ, Son BH, Ahn SH. Clinical implication of HER2/neu status in hormone receptor positive pure mucinous breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-08-22.