Abstract PO3-03-04: Metaplastic breast cancer: a single center retrospective study

Abstract Background: Metaplastic breast cancer (MBC) is a rare malignancy that accounts for up to 1% of all primary invasive breast carcinomas (BC). It is histologically heterogeneous, usually presents with a triple-negative phenotype and comprises low-grade and high-grade (HG) variants. HG variants have a higher risk of recurrence and a shorter disease-free and overall survival compared to other BC subtypes. Our research aimed to estimate the prevalence of HG-MBC among the Slovenian population and determine the characteristics of patients (pts) and tumours and the disease outcome. Patients and methods: Our retrospective study included pts diagnosed with HG-MBC at the Institute of Oncology Ljubljana from January 1983 until January 2021. Clinicopathologic characteristics such as tumour subtype, size and grade, nodal status, hormonal receptors (HR) and HER-2 status, lymphovascular invasion (LVI), tumour-infiltrating lymphocytes (TIL) and presence of germline BRCA mutation status were determined. The survival analyses were performed using the Kaplan-Meier method. The Cox proportional hazard model examined the association between risk factors and survival outcomes. Results: We evaluated 113 HG-MBC pts among a total of 27700 pts diagnosed with BC over 38 years (0.41%). The median age was 61.6 years (range 29.7 -93.9), majority of pts were postmenopausal (78.69%). The median follow-up was 15.5 years. The most common tumour subtype in our cohort was mixed MBC (53 cases, 46.9%), followed by MBC with mesenchymal differentiation (24 cases, 21.2%), squamous cell carcinoma (20 cases, 17.7%) and spindle cell carcinoma (16 cases, 14.2%). From the 113 evaluated pts, we obtained data about the stage in 105 pts, pathological tumour size in 100 pts, number of positive lymph nodes in 99 pts, HR status in 95 pts, HER2 status in 76 pts, grade in 97 pts, LVI in 85 pts, MIB-1 in 41 pts and TIL in 77 pts. At diagnosis, 17/105 pts (16.2%) had stage I disease, 59/105 pts (56.2%) stage II, 25/105 pts (23.8%) stage III and 4/105 pts (3,8%) stage IV. Most tumours were poorly differentiated (90/97, 92.2%) without LVI (60/85, 70.6%). Only 6/95 (6,3%) pts had positive HR, 7/76 (9.2%) pts had positive HER-2 status and 8/77(10.4%) pts intensive TIL. Overall, 13 pts were tested for BRCA germline mutation, among which only 1 (7,7%) had BRCA1 mutation. Modified radical mastectomy was the most frequent type of surgery (63.5%); 49.5% of the patients received radiotherapy. In total, 66/113 pts received CT: from 1983 to 2000, 16/36 (44.4%), and after 2000 50/77 (74.9%). In the first period, most pts received CMF (14/16; 87.5%) and anthracyclines and taxanes (27/50; 54%) in the second period. The disease progressed at 37 pts. At 19 pts, new malignancies were found. 55 pts died, 37 of them because of BC. Five- and 10-year disease-free survival (DFS) was 61.7% and 54.1%, while 5-and 10-year overall survival (OS) was 67.1 % and 56.7%, respectively. However, DFS and OS did not differ between the pre-2000 and post-2000 periods. The best outcome was found in pts with squamous cell carcinoma (5- and 10-year DFS 83.5% and 77.0% and 5-and 10- year OS 89.4 % and 83.0%). A subtype of MBC (squamous cell vs other) was the only predictive factor in multivariate analysis for both DFS (HR 0.21; 95% CI 0.05-0.92; p = 0.038 and OS (HR 0.27; 95%CI 0.09-0.78; p = 0.016), no association was seen between survival and tumour size, nodal status, stage, HR and HER2 status, grade, LVI and TILs. Visceral organs were the most common localization of distant metastases (21/37, 56.8%). Metastases in CNS occurred in 9/37 (24.3%) pts. Median OS after the first progression was only 0.9 years. Conclusions: The proportion of HG-MBC in our cohort of pts is 0.41%. Disease outcomes are poor; the 10-year OS of pts with early HG-MBC is only 56.6% and has not improved during the last decades. Squamous cell differentiation predicts a better outcome and is the only independent predictive factor of DFS and OS among HG-MBC pts. Citation Format: Maša Auprih, Gorana Gasljevic, Eva Setina, Anja Zizek, Simona Borstnar. Metaplastic breast cancer: a single center retrospective study abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-03-04.