Intraosseous myelolipoma.

Two patients with lesions in the acetabulum and femur, evident on imaging, were found to have normal marrow elements without a histopathological lesion on curettage of the acetabulum and resection of the femur, respectively. Radiographs on the first patient showed a sclerotic benign appearing acetabular lesion reminiscent of a fibro-osseous lesion while MRI showed the entire lesion to contain fat. The histological features were neither of a fibro-osseous lesion nor a lipoma, but only normal to mildly hypercellular marrow elements. The second patient had a long and expansive femoral osteolytic lesion which on biopsy showed cellular marrow. A subsequent stress fracture led to resection and prosthetic replacement. Microscopic findings revealed only cellular hematopoietic marrow with mature red and white cell lines, megakaryocytes, and a few mast cells devoid of trabecular bones. The microscopic features in the first patient raised the possibility of hematopoietic hyperplasia. Neither the radiographic nor MRI features were consistent with that diagnosis. The spectrum of imaging findings and microscopic appearances in both patients best fit the diagnosis of intraosseous myelolipoma.