17 Not a Collision Tumor: A case of a high grade sarcoma arising within a giant cell tumor of bone with aberrant keratin expression

Introduction/Objective Giant cell tumor of bone is a rare and commonly benign entity that rarely undergoes malignant transformation. Methods/Case Report Herein we describe a rare case of a giant cell tumor of bone with aberrant keratin expression undergoing malignant transformation into a high grade osteosarcoma in a 77 year-old female. The initial core biopsy showed focal areas of mononuclear cells and few giant cells. These mononuclear cells were positive for H3.3 G34W and p63, consistent with a giant cell tumor of bone. Adjacent to these neoplastic cells were a separate, high grade, cell population. These cells had keratin positivity, leading to an original diagnosis of a non-small cell carcinoma and “collision tumor”. Final resection showed mononuclear cells with a pale eosinophilic cytoplasm and numerous giant cells. There was also a distinct high grade sarcomatous cell population with fibroblastic/telangiectatic features with patchy areas consistent with woven bone/osteoid. These tumor cells are positive for H3.3 G34W, Cytokeratin AE1/AE3, 34BE12, OSCAR Keratin, and p40. Results NA Conclusion While a malignant giant cell tumor of bone with keratin expression is incredibly rare, care must be taken to not miss-identify this entity as a collision tumor involving a carcinoma.