MALIGNANT LOCALIZED FIBROUS TUMOR OF THE PERITONEUM ARISING IN ITS BENIGN COUNTERPART

We report a case of malignant localized fibrous tumor (LFT) of the per itoneum in a 78-year-old man. The tumor was pedunculated and the malig nant area completely surrounded by its benign counterpart, forming ''a nodule within a nodule'' appearance. Histologically, the latter area showed typical features of a benign LFT, white the former fulfilled th e criteria of a malignant LFT as defined by England et al.; i.e., one being rich in fibroblastic spindle cells with plump and atypical nucle i, hyalinized stroma and mitotic figures. Immunohistochemically, spind le cells in both portions of the tumor were positively stained for vim entin but completely negative for keratin (AE1/AE3), S-100 protein and desmin. There was a significant increase in proliferating cell nuclea r antigen (PCNA)-positive cells in the malignant portion (80%) in comp arison with the benign portion (45%). To date, we are not aware of any report on a malignant localized fibrous tumor in association with a b enign counterpart. The unique association, therefore, prompted us to r eport our experience, and suggests the possibility of malignancy devel oping in the benign localized fibrous tumor.