Although
mucinous carcinoma (MC) is considered a favorable histologic subtype of invasive
breast cancer (BC), a subset of MC is managed with
neoadjuvant therapy (
NAT). The clinical and pathologic features of MC following
NAT are not well characterized. The aim of this study is to characterize pathologic response in patients with MC treated with
NAT, including neoadjuvant endocrine
therapy (NET),
neoadjuvant chemotherapy (NCT), and
Herceptin-targeted NCT (H-NCT). We conducted a retrospective cohort study of 28 patients with MC who received preoperative adjuvant
therapy followed by resection from three institutions between 2010 and 2020. Demographic and clinical information were retrieved from the medical records. Pathologic review of the post
NAT resection specimens was performed including
tumor grading,
tumor size, staging,
residual tumor cellularity,
estrogen receptor (ER) and HER2 status. Nine (32 %) patients with ER+/HER2- MC received NET, 8 (29 %) ER+/HER2- MC were treated with NCT only and 11 (39 %) HER2+ MC received HER2-targeted NCT (H-NCT). The HER2+ MC patients were younger (45 vs. 64 years; p = 0.006). The HER2+ MC were of higher grade (p = 0.03) and more likely to be multifocal (p = 0.008). Only 2 of 28 (7 %) MC (both HER2+) showed complete pathologic response with residual acellular
mucin pools. Persistent mass-forming
mucin pools were present in 26 (93 %) cases. The
residual tumor cellularity was markedly reduced (≤5 %) in H-NCT treated MC (11/11, 100 %), followed by NET group (6/9, 67 %) and NCT only group (4/8, 50 %) (p = 0.011). Similarly, a higher rate of pathologic response (pCR/RCB-I) was observed in H-NCT (7/11, 64 %), followed by NET group (5/9, 56 %), and NCT only group (1/7, 13 %) (p = 0.053). Post-
therapy, all HER2+ MC were smaller than 2 cm and ypT size was significantly smaller in H-NCT group (11/11, 100 %) versus combined NET (5/9, 55 %) and NCT only groups (4/8, 50 %) (p = 0.029). We conclude that ER-/HER2+ and ER+/HER2-
mucinous carcinomas of the breast show robust pathological response to neoadjuvant HER2 targeted and endocrine
therapy, respectively. Our findings suggest that MC may show good response to endocrine
therapy.