E2259. Differential Considerations: Fibroblastic Osteosarcoma Presenting as a Giant Cell Tumor
  1. Zachary Thwing; Nova Southeastern Univ. Kiran C. Patel School of Medicine
  2. H. Thomas Temple; Nova Southeastern Univ. Kiran C. Patel School of Medicine
  3. Andrew Rosenberg; University of Miami Hospital
  4. James Banks; Nova Southeastern Univ. Kiran C. Patel School of Medicine
A 30-year-old female without significant past medical history presented with the chief complaint of increasing left knee pain for approximately 2 months. Radiographs of the knee were obtained, demonstrating a 4cm lucent lesion extending from the tibial metaphysis into the epiphysis. The lesion had a narrow zone of transition, lacked periosteal reaction, and was without cortical destruction. No pathologic fracture or soft tissue component. Given the classic radiographic presentation; a preliminary diagnosis of giant cell tumor was made. The patient was referred to an orthopaedic surgeon for cureting and bone grafting. Prior to surgery, biopsy and multiple frozen sections were obtained which did not demonstrate the classic appearance of a giant cell tumor, so permanent sections were obtained and deferred to expert review. Expert review by pathology ultimately demonstrated the lesion represented fibroblastic osteosarcoma, a rare osteosarcoma variant.

Educational Goals / Teaching Points
We will describe the classic presentation of giant cell tumors with respect to the clinical presentation, epidemiology, histology, and radiographic appearance on plain film, CT, and MRI. In doing so, we will discuss the imaging variants, commonly associated findings, potential imaging pitfalls, as well as differential diagnosis considerations. We hope to weave this discussion into an interesting case from our hospital that will allow for an engaging and memorable educational experience.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Giant cell tumors are relatively common bone tumors, comprising approximately 20% of benign bone neoplasms. The typical age of presentation is in early adulthood. The most common location of a giant cell tumor is within the knee, with the distal femur and proximal tibia occurring most frequently. The lesion abuts the articular surface and has a narrow zone of transition with non-sclerotic margins. The classic MRI presentation is a low to intermediate solid component on T1 that subsequently enhances. This helps to distinguish it from an aneurysmal bone cyst, which often occurs concomitantly with a giant cell tumor. Giant cell tumors also have heterogeneously high T2 signal intensity. Giant cell tumors are believed to arise secondary to the overexpression of a signaling pathway called the RANK/RANKL pathway. Histologically, giant cell tumors are characterized by the presence of diffuse osteoclastic giant cells.

Giant cell tumors are a relatively common entity that most radiologists will encounter in their career. By understanding the usual age groups, commonly associated findings on different imaging modalities and being armed with a good differential diagnosis, radiologists will be well equipped to help clinicians and their patients.