Desmoid Fibromatosis Fused With a Lipoma in the Upper Arm.

Lipoma, the most common mesenchymal tumor, often appears as a slow-growing mass in the musculoskeletal system (MSK). While generally non-invasive, their location can cause symptoms. Desmoid fibromatosis (DF), a rare and locally aggressive neoplasm, poses challenges in MSK system diagnosis and management due to its infiltrative nature. Despite lacking metastatic potential, DF has a high recurrence rate, classifying it as "intermediate, locally aggressive" in the WHO classification. Collaborative efforts among orthopedic surgeons, radiologists, and pathologists are crucial for accurate diagnosis and treatment planning for all tumors of the MSK system. This case report presents the first documented example of a DF within a lipoma, highlighting the challenges of diagnosing and treating musculoskeletal tumors.