GIANT-CELL TUMOR IN THE SKULL OF A 9-YEAR-OLD CHILD - IMMUNOHISTOCHEMISTRY TO CONFIRM A DIAGNOSIS RARE FOR AGE AND SITE

Giant cell tumor of the bone is usually located within the epiphysis o f a long bone, the majority of the lesions occurring in the third and fourth decades of late. We report an unusual case of giant cell tumor (GCT) arising in the parietal skull bone of a 9-year-old girl. The tum or exhibited histologic findings typicalfor GCT, with conspicuous intr avascular giant cells. Based on microscopic features, not only conditi ons like aneurysmal bone cyst or bone changes associated with hyperpar athyroidism but also tumors such as cltondroblastoma or osteosarcoma h ad to be considered. Immunohistochemistry revealed strong reactivity o f the tumor giant cells and normal bone osteoclasts with CD68 but nod Mac-387; tumor stromal cells were uniformly negative for both. The str omal cells exhibited two immunohistochemicably distinct phenotypes. On e, involving 50-80% of the tumor cells, exhibited negative lysozyme st aining with positivity of proliferating cell nuclear antigen (PCNA) in about 30 % of the nuclei. The other showed reactivity with lysozyme b ut negative PCNA staining. Immunohistochemistry thus helped to disting uish chondroblastoma and osteosarcoma, in which lysozyme positivity wo uld reside in macrophages but not within stromal cells. Instead, chond roblastoma would exhibit protein S-100 positivity in the tumor cells. The biological behavior of GCT is difficult to predict based on morpho logy alone, although the malignant potential seems to rest in the stro mal cells rather than the giant cells. Specifically, in reported cases , the intravascular occurrence of giant cells in GCT is not associated with an increased incidence of metastasis.