Multiple skeletal muscle metastases from a high-grade sinonasal myoepithelial carcinoma: a case report

Introduction and importance: Myoepithelial carcinoma is a rare salivary-type malignancy that metastasizes to distant sites in about one quarter of patients, mainly to the lungs. Although skeletal muscle accounts for nearly half of the body’s mass, it rarely hosts metastases owing to continuous contraction, variable perfusion, and local anti-tumor factors. Multiple, bilateral skeletal-muscle metastases from a sinonasal myoepithelial carcinoma have not, to our knowledge, been reported. Presentation of case: A 79-year-old woman underwent R0 resection and adjuvant radiotherapy in 2021 for a high-grade sinonasal myoepithelial carcinoma. In 2025, three metachronous pulmonary metastases were treated with stereotactic radiotherapy. In 2026, 18F-fluorodeoxyglucose positron emission tomography/computed tomography ([ 18 F]FDG-PET/CT) showed recurrent oligometastatic disease: two intramuscular thigh deposits and a left thoraco-dorsal subcutaneous deposit, all resected en bloc. Histology confirmed metastatic myoepithelial carcinoma at each site, with involved (R1) intramuscular margins. Margin-directed radiotherapy followed, and at 6 months, restaging CT showed no evidence of disease. Clinical discussion: This case adds three observations. First, the main diagnostic challenge was distinguishing adenoid cystic-like myoepithelial metastases from adenoid cystic carcinoma; the absence of MYB, EWSR1, and PLAG1 rearrangements in the primary supported a myoepithelial origin. Second, an intramuscular mass in this setting must be biopsied to exclude alternative diagnoses. Third, in repeat oligometastatic disease in a fit older patient without an actionable target, surgery provided histological confirmation, symptom relief, and local control with minimal morbidity. Conclusion: In myoepithelial carcinoma, a new intramuscular mass warrants imaging and biopsy. Image-guided metastasectomy with margin-directed radiotherapy may be considered in carefully selected patients, although long-term benefit remains unproven.