SOLITARY FIBROUS TUMOR OF SOFT-TISSUE - A REPORT OF 15 CASES, INCLUDING 5 MALIGNANT EXAMPLES WITH LIGHT-MICROSCOPIC, IMMUNOHISTOCHEMICAL, AND ULTRASTRUCTURAL DATA

We describe 15 soft tissue solitary fibrous tumors (SFTs) occurring in patients 24 to 78 years old (average, 50.6 yr). Ten tumors were benig n and arose in the head and neck area (three tumors), thigh (two), vul va (two), upper arm (one), lower leg (one), and retroperitoneum (one). Five tumors were histologically malignant and arose in the thigh (two ), abdominal wall (one), buttock (one), and retroperitoneum (one). All of the tumors were grossly well circumscribed. The benign tumors meas ured from 2 to 10 cm (average, 4.8 cm) and the malignant ones from 3 t o 5.5 cm (average, 4.3 cm) in greatest diameter. Microscopically, the benign tumors showed areas of hypercellularity with variable amounts o f collagenous and myxoid stroma; one had amianthoid fibers. The malign ant tumors were composed of cytologically atypical cells enmeshed in a collagenous or myxoid extracellular matrix. Ultrastructural study of three benign and three malignant tumors showed fibroblastic differenti ation; one benign tumor showed myofibroblastic differentiation. Immuno histochemically, all of the tumors examined were immunoreactive for vi mentin, and seven of nine were positive for CD34, including all of the malignant ones. There was focal staining for muscle actin in two beni gn tumors and for Leu-7 in one benign tumor; there was no staining for cytokeratin, desmin, S-100 protein, epithelial membrane antigen, or s mooth muscle actin in any of the examined tissues. Follow-up was avail able for eight patients for 6 to 21 months (average, 12 mo). No tumor recurred locally or metastasized. The SFTs reported herein support the experiences of others who recently described these tumors in the soma tic soft tissues, In addition, our series highlights the occurrence of malignant SFTs in the soft tissues, SFTs should be separated from oth er spindle cell sarcomas, with which they can be confused.