A retrospective, descriptive analysis of a single-center experience with patients with metaplastic breast cancer.
Authors: Dempsey Naomi , Rubens Muni , Advani Pooja Prem , Sandoval-Leon Ana Cristina , Carcas Lauren , Ahluwalia Manmeet Singh , Mahtani Reshma L. 
Published: June 2024

e13173 Background: Metaplastic breast cancer (MpBC) is a subtype of breast cancer (BC) that is commonly triple negative (TN) and chemotherapy (CT) resistant. Despite CT resistance, standard of care therapy is administered according to receptor status, regardless of metaplastic histology. Randomized data regarding optimal treatment of this rare histologic subtype are lacking. Methods: This was a retrospective analysis of MpBC patients (pts) treated at Miami Cancer Institute (MCI) between 2017 and 2021. Pts were identified using COTA real-world Analytics, an analytics platform enabling rapid investigation of longitudinal real-world data. Pt demographics, pathologic characteristics, staging, treatment, and survival outcomes were collected and analyzed. Results: 46 pts with MpBC were identified. Median age at diagnosis was 60.5 years, and 83.7% were post-menopausal. 84.8% were White, 10.9% were Black, and 54.3% were Hispanic (all White). 80.4% (n=37) had TNBC (ER < 1% and HER2 negative by ASCO CAP guidelines). 13.0% of pts (n=6) had spindle cell morphology and these pts had numerically lower 5-yr overall survival (OS) compared to those with non-spindle cell morphology; (83.3%; 95% CI 53.5%-100% vs 91.7%; 95% CI 82.7%-100%). Of 21 pts (45.7%) who underwent germline genetic testing, a pathogenic variant was detected in 2 (9.5%); (BRCA2, PALBB2). At diagnosis, 93.5% (n=43) had early-stage disease, 2.2% (n=1) had de novo metastatic breast cancer (MBC), 2 were unknown. Of the early-stage pts, 12 (27.9%) had stage 1, 19 (44.1%) stage 2, and 6 (14.0%) stage 3 BC. Of 43 pts with treatment records available, 34 (79.1%) received CT and 20 (46.5%) received anthracycline-based CT in the early-stage setting. Of those who received CT, 50% (n=17) were treated in the neoadjuvant setting, and 2 pts (11.8%) achieved pathologic complete response (pCR). Median distant recurrence free survival (DRFS) and OS were not reached in the overall cohort. Two-year OS was 90.5% (95% CI 81.6%-99.4%) and two-year DRFS was 78.9% (95% CI 66.6%-91.2%). Of the 43 pts with early-stage MpBC at diagnosis, 11 (25.6%) developed MBC with lung as the most common site of metastasis. Only 15.2% (n=7) of tumors underwent somatic mutation analysis and 71.4% (5/7) were PIK3CA-mutated. Of the pts who developed MBC, median progression-free survival with cytotoxic chemotherapy was 9.0 months (95% CI 5.0-13.0). Conclusions: MpBC represents an aggressive phenotype of BC and optimal guidance regarding treatment is lacking. Consistent with published literature, a high proportion of MpBC in this study were TN (>80%). Our dataset represents the largest cohort to date of Hispanic pts with MpBC. Despite the poor outcomes historically reported with MpBC, median DRFS and OS were not reached in this cohort, with short follow up. Further prospective randomized data on the optimal treatment of MpBC is warranted and a larger multi-institution review is planned.