Selected case from the Arkadi M. Rywlin International Pathology Slide Seminar: desmoplastic fibroblastoma (collagenous fibroma).

Desmoplastic fibroblastoma (collagenous fibroma) is a rare and recently described benign soft tissue tumor. It is more common in men, median age is 50 years, the history is usually long, and the usual size is around 3 cm. Most lesions are subcutaneous but approximately 25% involve skeletal muscle. Histologically, they are sparsely cellular with stellate and spindle-shaped fibroblasts-myofibroblasts that are separated by a collagenous or myxocollagenous matrix. Mitoses are minimal or absent. Tumor cells are focally positive for muscle-specific and alpha smooth muscle actins, and rarely, for keratins. The S-100 is usually negative but a very few cases have been positive. It can be confused with one of the fibromatoses, myxomas of various kinds and neural tumors. It neither recurs nor metastasizes.