Myoepithelial carcinoma: Defining natural history and therapeutic outcomes in a rare malignancy.

e18143 Background: Myoepithelial carcinoma (MEC) is a neoplasm derived from myoepithelial cells and characterized by an infiltrative growth pattern. These are very rare malignancies with limited published data describing outcomes. We sought to define the natural history of MEC and to identify best available treatments. Methods: In this retrospective series, we identified 70 patients with MEC seen at our institution from 1997-2025. Electronic medical records were reviewed to determine the patient and tumor characteristics, treatment regimens and response, and outcomes. The Kaplan Meier method was used to estimate survival, and log-rank tests were used to compare groups. Results: In our institutional cohort of 70 patients, MEC was most common in middle-aged individuals (median age = 52 years), with a predilection for head and neck sites (54/70, 77%). There was a slight predilection for male sex (40/70, 57%). The median overall survival (mOS) in our cohort was 87.2 months. Of 24 patients who developed metastatic disease, the predominant location was lung (n = 15, 62.5%) followed by bone (n = 11, 45%). For patients with localized disease (n = 66), the median recurrence-free survival (mRFS) was 55.4 months. The mRFS was not significantly different in patients who had resection alone (n = 39, mRFS = 60.0 months) compared with those treated with adjuvant radiation and/or chemotherapy (n = 27, mRFS = 51.1 months, p = 0.70). The primary systemic therapy utilized was platinum-based with few patients receiving doxorubicin or gemcitabine-based regimens. There were no significant clinical responses noted with systemic therapy. The median progression-free survival (mPFS) in patients with metastatic disease who received any systemic therapy was 3.1 months (n = 16). No significant difference in mOS was observed based on age, though there was a trend toward worse outcomes in younger patients (mOS for <30 years = 62.5 months, mOS for >30 years = 93.5 months, p = 0.88). The sample size of patients in the younger age group (n = 11) was small and only three patients in that subset were free of disease for >15 years. Conclusions: In our institutional experience with MEC, most patients were middle-aged with head and neck as the most common primary site. Outcomes were generally worse for younger patients. The lack of significant difference between the mOS between these age groups is attributed to a small sample size of patients in the younger age group with a wide confidence interval. Outcomes with currently available systemic therapy for metastatic disease were uniformly poor, emphasizing the need to develop novel treatment regimens moving forward. Future work is needed to discern the impact of specific genomic alterations (e.g. EWSR1-KLF15 fusion) on tumor biology and response to therapy.