Infiltrating hybrid mesenchymal tumor of skeletal muscle showing lipomatous, hemangiomatous, leiomyomatous and osseous features - An unusual soft tissue tumor providing insight into the pathogenesis of lipoma variants.

Infiltrating angiolipoma and osteolipoma of the hand are rare. A 40-year-old man presented with slowly enlarging swelling of his right hand for two and half years without functional deficit but it became painful with slight limitation of movement for the past few months. Plain radiograph showed a large soft tissue swelling with specks of calcifications. Ultrasonography and magnetic resonance imaging revealed an infiltrative mass in the right palm deep to the flexor tendons. As biopsy suggested infiltrative angiolipoma of skeletal muscle, debulking of the tumor was performed. The tumor showed coexistence of all four mesenchymal elements, fat, blood vessel, smooth muscle and bone in various combinations in the form of angiolipomatous, osteolipomatous, ossified intramuscular hemangiomatous, myolipomatous and angiomyolipomatous patterns throughout the entire tumor. Small meningothelial-like whorls of spindle cells were focally seen, some showed intramembranous ossification forming small woven bony spicules in their centers. There were no atypical cells or lipoblasts. Staining for CDK4, MDM2, p16, HMB45 and Melan A was negative. The diagnosis was "infiltrating hybrid mesenchymal tumor of skeletal muscle showing lipomatous, hemangiomatous, leiomyomatous and osseous features". The fairly even admixture of the various components supports the neoplastic participation of each individual element. Hybrid mesenchymal tumor most probably originates from multipotent neoplastic cells showing multidirectional differentiation.