An Unusual Presentation of Low Grade Central Osteosarcoma in the Distal Femur of a Sixty-eight Year Old Male Mimicking Fibrocartilaginous Mesenchymoma

Low grade central osteosarcoma is a rare intramedullary bone tumor that presents a diagnostic challenge due to its radiographic and histologic overlap with other low grade intraosseous lesions. Here we report a case of a 68 year old male presenting with local pain in the right distal thigh. An MRI study showed a 4.8 cm lesion in the metadiaphysis of the distal femur with extension into the cortex and aggressive periosteal reaction. Multiple biopsies were performed in an attempt to reach a conclusive diagnosis: 1) A CT guided FNA and biopsy revealed a low nuclear grade spindle cell neoplasm with new bone formation; 2) Subsequent core biopsy showed bland cartilage with no atypical features and reactive new bone formation; 3) An excisional biopsy was reported as a matrix (hyaline cartilage and bone) producing neoplasm showing features most consistent with fibrocartilaginous mesenchymoma. Due to uncertain malignant potential, the patient underwent complete resection of the distal femur with the final diagnosis of low grade central osteosarcoma.  Molecular studies performed showed lack of CPM amplification. This case report illustrates the diagnostic challenge of an atypical case of low grade central osteosarcoma.  It required multiple procedures, molecular studies, and correlating the histology to the radiology and clinical picture to arrive at the correct diagnosis. [N A J Med Sci. 2016;9(2):78-82.   DOI:  10.7156/najms.2016.0902078]