The clinical presentation is consistent with a intramuscular lipoma. The radiograph shows a soft tissue density on the left side. The axial MRI images reveals a mass within the gluteus maximus that is bright on T1 and dark on the STIR image. The pathology shows lipoblasts and muscle infiltration which is characteristic of these tumors. The cited reference by Damron et al reviews common soft tissue masses seen by orthopaedic surgeons. They outline an algorithm for evaluation and management of both benign and malignant tumors. Treatment for intramuscular lipomas involves surgical resection only if symptomatic, as these tumors are benign. Matsumoto et al reviewed MRI findings of intramuscular lipomas and found that imaging characteristics varied from a homogeneous mass as commonly seen in an ordinary (superficial) lipoma, to a large, inhomogeneous lesion with an infiltrative margins. Infiltrative margins did not indicate malignancy.
Damron TA, Beauchamp CP, Rougraff BT, Ward WG Sr. Soft-tissue lumps and bumps. JBJS (American) 85:1142-1155 (2003)
PMID:15116652 (Link to Abstract)
Matsumoto K, Hukuda S, Ishizawa M, Chano T, Okabe H. MRI findings in intramuscular lipomas. Skeletal Radiol. 1999 Mar;28(3):145-52.
PMID:10231912 (Link to Abstract)