Malignant localized fibrous tumor of the peritoneum arising in its benign counterpart

We report a case of malignant localized fibrous tumor (LFT) of the peritoneum in a 78-year-old man. The tumor was pedunculated and the malignant 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, while the former fulfilled the criteria of a malignant LFT as defined by England et al.; i.e., one being rich in fibroblastic spindle cells with plump and atypical nuclei, hyalinized stroma and mitotic figures. Immunohistochemically, spindle cells in both portions of the tumor were positively stained for vimentin but completely negative for keratin (AE1/AE3), S-100 protein and desmin. There was a significant increase in proliferating cell nuclear antigen (PCNA)-positive cells in the malignant portion (80%) in comparison with the benign portion (45%). To date, we are not aware of any report on a malignant localized fibrous tumor in association with a benign counterpart. The unique association, therefore, prompted us to report our experience, and suggests the possibility of malignancy developing in the benign localized fibrous tumor.