Undifferentiated Pleomorphic Sarcoma: A Diagnosis of Exclusion

A 43-year-old male patient presented with a progressively enlarging mass in his right lateral tibia over the course of one month. The mass measured 4 x 7 cm in size on examination. Aspiration of the lesion was performed, revealing 30 cc of dark blood, leading to an initial diagnosis of hematoma. Three months later, the mass had enlarged with mild pain. Physical examination revealed a 12 x 9 cm fluctuant mass on the right lateral lower leg, which was warmer than surrounding skin and tender upon palpation. Subsequent imaging was ambiguous, with a broad differential diagnosis consisting of expanding hematoma, abscess, neoplasm, and arteriovenous malformation (AVM). A biopsy was performed revealing pleomorphic cells consistent with an undifferentiated pleomorphic sarcoma (UPS). This case report highlights the challenges of diagnostic workup, management, and follow-up of an unusual presentation of a UPS.