Undifferentiated Pleomorphic Sarcoma Mimicking a Refractory Thigh Abscess in a Young Adult: A Diagnostic Pitfall.

Undifferentiated pleomorphic sarcoma (UPS) is a high-grade sarcoma typically seen in older adults; its occurrence in young adults is rare and poses significant diagnostic challenges. We report the case of a 21-year-old female who initially presented with a presumed infectious lesion that proved refractory to standard antibiotic therapy and incision and drainage. Progressive enlargement prompted surgical excision, with pathology demonstrating FNCLCC grade 3 UPS. Histopathologic evaluation revealed a pleomorphic high-grade neoplasm with extensive necrosis and high mitotic activity, while immunohistochemical and molecular studies, including negative MDM2 amplification, supported the diagnosis of UPS. Staging studies demonstrated no evidence of metastatic disease. The patient subsequently underwent wide local re-excision with negative margins, and postoperative MRI demonstrated no residual enhancing mass. During subsequent follow-up, the patient continued oncologic care at Memorial Sloan Kettering Cancer Center. Documentation from December 2025 reported multiple pulmonary lesions identified on surveillance CT imaging. Subsequent biopsy confirmed metastatic sarcoma involving the lungs. The patient was subsequently enrolled in an immunotherapy-based clinical trial at the outside institution. The patient also continued routine follow-up at our institution every six months. Local examination of the right posterior thigh surgical site demonstrated no evidence of local recurrence. At the most recent documented follow-up in March 2026, the patient remained under active multidisciplinary oncologic management with ongoing surveillance. This case highlights the importance of considering soft tissue sarcoma in atypical or nonresolving soft tissue lesions to avoid delays in diagnosis and management.