Giant cell tumor of the clivus misdiagnosed as calcifying fibrous tumor

Abstract

Abstract Introduction/Objective Giant cell tumors of bone (GCTs) are generally benign, locally aggressive lesions accounting for approximately 4% of primary bone tumors. GCTs of the skull are rare and constitute <1% of all reported GCTs as they typically occur in the epiphysis of long bones. Specifically, in the clivus location, these tumors are exceedingly rare with fewer than 10 articles in English literature. The purpose of this report is to describe a case of GCT in this unusual location that was misdiagnosed as giant cell reparative granuloma on biopsy and subsequently calcifying fibrous tumor on resection. Methods/Case Report A 34-year-old male presented with history of headache, visual disturbance, epistaxis, and nausea. MRI demonstrated a large soft tissue mass eroding the clivus. A biopsy was interpreted at an outside institution as giant cell reparative granuloma. The patient was treated with denosumab and subsequently underwent resection of the mass due to worsening symptoms. Post-treatment pathology was interpreted as calcifying fibrous tumor. The patient presented to our institution for further management and outside pathology was reviewed. Histologically, the initial tumor was composed of a bland spindle cell proliferation with admixed osteoclast-type giant cells. In this location, the differential diagnosis includes Brown tumor, giant cell reparative granuloma, and, rarely, GCT. HG34W immunostain was positive in the tumor cells confirming the diagnosis of GCT. The post-therapy specimen showed abundant woven bone formation and a lack of giant cells. Post-denosumab GCT are known to show these features. Follow up imaging showed a slow growing moderately enhancing mass at the resection site. Patient is alive with disease 36 months after initial diagnosis. Results (if a Case Study enter NA) NA Conclusion With rare exceptions, bone tumors frequently follow generalized patterns of involvement. Pathologists need to be aware of these exceptions. Appropriate panel of immunohistochemical stains help clinching the right diagnosis, especially in unusual clinical settings.