SOLITARY FIBROUS TUMOR - CONSISTENT CD34 IMMUNOREACTIVITY AND OCCURRENCE IN THE ORBIT

The distinction of solitary fibrous tumors (SFTs) from histologically similar neoplasms relies heavily on a characteristic microscopic appea rance. No discriminating ultrastructural or immunohistochemical featur es are known. We evaluated 22 SFTs and 118 other tumors often consider ed in the differential diagnosis for immunoreactivity using a monoclon al antibody directed against the human hematopoietic progenitor cell a ntigen, CD34. All the SFTs (22 of 22, 100%) demonstrated strong CD34 i mmunoreactivity, irrespective of tumor site and histologic grade. Stro ng and generalized CD34 positivity was also found in most dermatofibro sarcoma protuberans (11 of 12, 92%) and occasional smooth-muscle tumor s (leiomyomas 2 of 11, 18%; leiomyosarcomas 2 of 11, 18%). Variable nu mbers of CD34 positive cells were present in all neurofibromas (9 of 9 , 100%) and focally present in most schwannomas (8 of 9, 89%). Some of the hemangiopericytomas (7 of 16, 44%) exhibited CD34 immunoreactivit y, however, generally with weak intensity and patchy distribution. CD3 4 immunoreactivity was not observed in mesotheliomas (0 of 20, 0%), sy novial sarcomas (0 of 13, 0%), fibrosarcomas (0 of 12, 0%), or spindle -cell thymomas (0 of 5, 0%). We conclude that CD34 immunoreactivity is a sensitive marker for SFT and, in conjunction with an appropriate im munohistochemical panel, may be useful in discriminating SFTs from oth er histologically similar neoplasms. The observation that some mesench ymal stromal cells and SFTs share a CD34-positive immunophenotype sugg ests a histogenetic relationship. The inclusion in this study of two c ases of SFTs arising in the orbit establishes another site of origin f or this tumor and provides further support for a mesenchymal histogene sis.