Incidence and histological structure of the storiform pattern in benign and malignant fibrous histiocytomas

A starlike arrangement of cells and fibers, the "storiform pattern", was found to be a typical, but not obligatory, histological feature of benign and malignant fibrous histiocytomas. In 155 benign fibrous histiocytomas storiform structures were missing in 29 cases, chiefly of the fibroblastic type comparable with classical "dermatofibroma". 12 of 70 malignant fibrous histiocytomas did not reveal storiform structures, especially the cellular pleomorphic variant, i.e. the classical pleomorphic sarcoma. Storiform structures were either small and highly cellular with few fibers (collagen type III), or larger, less cellular, but with abundant fibers (collagen type I). There was no sharp demarcation between these two extremes, but many transitional structures or patterns were seen. The histiocytic nature of the cells was demonstrated in both variants of storiform structures by immunhistochemical methods on paraffin embedded material. Alpha 1-antichymotrypsin was especially valuable in this respect.