Fibroblastic Type Osteosarcoma of the Ulna: a Case Report of a Tumor in a Rare Location with Atypical Imaging Findings

Abstract

The ulna is a rare site of origin for osteosarcoma, and purely osteolytic osteosarcomas are uncommonly noted on conventional radiographs. We present a patient with a lytic lesion of the distal ulna for which imaging findings suggested an aneurysmal bone cyst. The lesion was histologically confirmed to be a fibroblastic osteosarcoma. Osteosarcoma is the second most common primary malignant bone tumor, accounting for approximately 15% of all primary bone tumors confirmed at biopsy (1). Osteosarcomas most frequently affect long bones, particularly around the knee, and rarely the ulna (1, 2). On radiographs, osteosarcomas classically appear as aggressive destructive lesions with a mixed osteolytic and sclerotic pattern, associated periosteal reaction, and soft-tissue mass. The purely osteolytic form of osteosarcoma occurs in about 10% of all cases (3). The differential diagnosis of lytic and expansive bone lesions includes solitary bone cysts, aneurysmal bone cysts, giant cell tumors, enchondromas, and telangiectatic osteosarcomas (4). We present a case of fibroblastic osteosarcoma, with an accompanying pathologic fracture, which arose from the distal ulna and was initially suspected to be an aneurysmal bone cyst based on its features on conventional radiographs and magnetic resonance (MR) images.