Myelolipoma mimicking osteosarcoma in the distal femur

INTRODUCTION AND IMPORTANCE: Periosteal reactions indicate malignant bone tumors, including osteosarcoma; establishing an accurate diagnosis is key to determining the most appropriate treatment strategy. We describe a rare case of myelolipoma in the distal femur metaphysis with massive extraskeletal lesions and periosteal reactions. CASE PRESENTATION: A 25-year-old woman was referred to our hospital to treat a gradually expanding mass around her knee that grew to the size of a baby's head. She had a history of hydrocephalus caused by congenital cytomegalovirus infection and was bedridden for life. Radiography showed a prominent osteoblastic rim and osteolytic lesion with a moth-eaten appearance. Osteosarcoma was suspected due to excessive extraskeletal invasion and periosteal reactions. T1- and T2-weighted magnetic resonance images showed a high-signal-intensity homologous lesion. Biopsy specimens contained adipose and hematopoietic tissues. A myelolipoma was diagnosed. Due to her fragility, surgical intervention was suspended. Two years after diagnosis, the tumor size did not change. CLINICAL DISCUSSION: Myelolipomas are benign tumors that typically arise from the adrenal gland and rarely develop in the extremities. This type of tumor typically does not cause any tumor-related symptoms or endocrine disturbances and has been reported as a type of incidentaloma. To effectively manage myelolipoma patients, differential diagnosis of tumors mimicking malignant bone tumors is important. CONCLUSION: We successfully managed a destructive ectopic myelolipoma in the distal femoral metaphysis, with massive extraskeletal lesions and periosteal reactions. Clinicians should appropriately differentiate myelolipoma from tumors mimicking malignant bone tumors.