Myxoid solitary fibrous tumor: a clinicopathologic study of three cases.

While focal myxoid areas are occasionally observed in solitary fibrous tumors, neoplasms of this type exhibiting extensive myxoid change are considered exceedingly uncommon. Due to their rarity, the biologic behavior of myxoid solitary fibrous tumor has not been determined. Three cases of myxoid solitary fibrous tumor are described in order to better characterize the clinical and pathologic features of this uncommon variant of solitary fibrous tumor. The tumors occurred in one man and two women, with ages of 37, 47, and 58 years, respectively. Sites of involvement included the retroperitoneum, pelvis, and soft tissue of the neck. Histologically, all cases were characterized predominantly by the presence of myxoid stroma comprising 70% to 100% of the tumor. The tumor cells were predominantly spindled in all cases, and arranged randomly, in loose fascicles, or in anastomosing strands imparting a microcystic/reticular appearance. The lesional cells had a bland cytologic appearance and low mitotic count. All tumors lacked necrosis and areas of increased cellularity. By immunohistochemistry, all cases were positive for CD34, CD99, and bcl-2, and negative for keratin, epithelial membrane antigen, desmin, actin, smooth muscle actin, and S-100 protein. To date, all cases have followed a benign course without evidence of recurrence or metastasis with a follow-up duration ranging from 50 to 87 months. The data suggest that myxoid solitary fibrous tumors are associated with an indolent clinical course and favorable prognosis.