Malignant giant cell tumor of soft parts presenting as a skin tumor.

Malignant giant cell tumor of soft parts is a rare neoplasm that histologically resembles a giant cell tumor of bone. It has a distinctive multinodular growth pattern and is composed of numerous osteoclast-like giant cells, histiocytes, and fibroblasts. Although this tumor is usually found in deep soft tissues, a superficial form has been described in the subcutaneous tissue and fascia. The authors report two patients, aged 75 and 78, with malignant giant cell tumors presenting as ulcerating skin nodules of the arm and foot. The tumors were relatively small, measuring less than 3.5 cm in greatest dimension, and involved the entire dermis and subcutaneous tissue. The clinical differential diagnoses included Kaposi's sarcoma, melanoma, and hematoma. Dermatopathologists and dermatologists should be aware of this entity to avoid confusion with other benign and malignant neoplasms that may contain multinucleated giant cells. The distinguishing histologic and immunohistochemical features of this tumor are discussed.