A Rare Case of Recurrent Sarcoma Presenting as an Anterior Mediastinal Mass: Case Report.

Introduction: Soft tissue sarcomas (STS) represent a heterogenous group of mesenchymal neoplasms that frequently arise in the extremities. In cases of disease recurrence, the lungs are the most commonly affected site. We report a rare case of a previously treated lower extremity STS with subsequent delayed recurrence in the anterior mediastinum, an uncommon site for distant metastasis. Case Presentation: Our patient was a 64-year-old woman with a history of a left medial thigh pleomorphic/spindle cell sarcoma previously treated with wide local excision and adjuvant radiation therapy in 2013. She presented in 2025 with a 2-month history of nonproductive cough and 1 day of left lower extremity swelling. CT chest revealed an 18.7 cm heterogeneous anterior mediastinal mass, and tissue sampling was consistent with recurrence of an undifferentiated pleomorphic sarcoma with spindle cell features. She subsequently underwent surgical resection of the mediastinal mass. Due to positive margins, adjuvant radiation therapy is currently being considered. Conclusion: This case highlights an atypical pattern of late recurrence of spindle cell sarcoma to the mediastinum - an uncommon site for metastatic spread - occurring 12 years following initial treatment. It underscores the potential need to reevaluate current surveillance recommendations beyond the standard 5-year period. Furthermore, it emphasizes the importance of investigating possible alternative mechanisms underlying STS recurrence.