Pleomorphic leiomyosarcoma of the soft parts: a reassessment by histology and immunohistochemistry of pleomorphic soft tissue sarcomas.

Histologically it is often difficult to differentiate malignant fibrous histiocytoma (MFH) from leiomyosarcoma. Sixty-three cases formerly diagnosed as pleomorphic soft tissue sarcomas were examined by immunohistochemistry. We identified three different types according to its positivity for the myogenic markers: A) positive cells with a fascicular pattern, 13 cases (26%); B) a small number of scattered positive cells, 29 cases (57%); C) negative for markers, 9 cases (17%). We reassessed 51 MFHs as follows: A type as pleomorphic leiomyosarcoma, B+C types as MFH. Pleomorphic leiomyosarcomas were also positive for collagen type IV around the compactly arranged individual tumor cells. An imunohistochemical analysis and detection of fascicular structures are thus considered to be necessary in order to distinguish between leiomyosarcoma and MFH.