Secondary malignant giant cell tumor of bone due to malignant transformation 40 years after surgery without radiation therapy, presenting as fever of unknown origin: a case report.

BACKGROUND Malignant transformation of giant cell tumors of bones, that is, secondary malignant giant cell tumor of bone, is rare. The most common symptoms are local pain and swelling. There are no prior reports of giant cell tumor of bone with fever of unknown origin at the onset. Here we present a case of a secondary malignant giant cell tumor of bone due to malignant transformation 40 years after surgery without radiation therapy, presenting as fever of unknown origin. CASE PRESENTATION A 75-year-old Asian man presented with a 3-week history of continuous pyrexia and left knee pain and swelling. He had been diagnosed at age 35 years with a giant cell tumor of bone of his left distal femur and underwent bone curettage and avascular fibula grafting at that time. Postoperative radiation therapy was not performed. He remained recurrence-free for 40 years after surgery. At age 75, histopathological findings suggested a secondary malignant giant cell tumor of bone. The tumor specimen expressed tumor necrosis factor-α. Neoplastic fever was suspected, and a naproxen test was conducted. His pyrexia showed immediate resolution. Surgery was performed under a diagnosis of a secondary malignant giant cell tumor of bone with neoplastic fever. His pyrexia and inflammatory activities diminished postoperatively. CONCLUSIONS This is the first reported case, to the best of our knowledge, of the detection of a secondary malignant giant cell tumor of bone based on fever of unknown origin after long-term (40 years) follow-up. After curettage and bone grafting, giant cell tumor of bone may transform to malignancies within a few years or even decades after surgery. Therefore, meticulous follow-up is essential. The fever might be attributable to the tumor releasing inflammatory cytokines. Not only pain and swelling but also continuous pyrexia may suggest the diagnosis of a secondary malignant giant cell tumor of bone.