Answer: Low grade fibromyxoid sarcoma

Histology: The specimen consists of a fibrous tumor with alternating zones of myxoid and dense collagenous fibrous tissue. Within the myxoid areas the cells are composed of bland fibroblastic cells without atypia or appreciable mitotic activity. The myxoid areas have a prominent thin walled vascular network. The more collagenized areas show similar bland fibrous tissue with denser collagen. The lesion is not organized into fascicles, nor does it have a storiform appearance. Focally, there are small rounded nodules of fibrinoid collagen surrounded by rounded epithelioid fibroblasts.

Discussion: The histological features in this case are classic for low grade fibromyxoid sarcoma, also known as Evans’ tumor. In contrast to fibromatosis, the lesion lacks well organized, long, intersecting fascicles of fibrous tissue. Dermatofibrosarcoma protuberans (DFSP) is characterized by a more cellular infiltrate and a characteristic storiform pattern. The histological features in this case, with the alternating hypocellular zones and more cellular nodules, is typical, as are the vasculature arcades of vessels within the more myxoid areas. Approximately 40% of low grade fibromyxoid sarcomas have collagen rosettes as seen in the current case. In the past, these lesions were designated separately as hyalinizing spindle cell tumor with giant rosettes. However, currently it is considered that low grade fibromyxoid sarcomas with and without giant rosettes are identical in terms of their behavior and should be considered as the same entity. Although the initial study of this lesion by Evans suggested that these lesions were fairly aggressive, these cases initially were often diagnosed as benign and not treated accordingly. With more appropriate surgery, recurrences occur in less than 10% with metastases seen in only approximately 6% of cases.