Solitary fibrous tumor: A study of cytologic features of six cases diagnosed by fine-needle aspiration

Solitary fibrous tumor (SFT) is a spindle-cell neoplasm most often presenting as a pleural-based tumor but increasingly recognized in other locations. Few reports have described the cytologic features of SFTs. Six cases of SFT diagnosed by fine-needle aspiration (3 pleura, 2 retroperitoneum, and 1 orbit) were identified in the Mayo Clinic files. The smears (Papanicolaou-stained) and corresponding histologic specimens were reviewed. Immunohistochemical staining for CD34 was performed in all cases. The cytologic findings were similar in all cases. The tumor cells were oval to polygonal, with cellularity ranging from scant to moderate. The background contained irregular ropy fragments of collagen and a few inflammatory cells. Most cells were dispersed singly, but all cases contained irregular, loose aggregates of cells enmeshed in a collagenous matrix. The nuclei were uniformly bland, with evenly distributed, finely granular chromatin. All cases were immunoreactive for CD34. SFT has distinctive cytologic features that allow diagnosis in cytologic specimens with the help of appropriate immunocytochemical stains on accompanying tissue biopsy specimens. Distinctive cytologic findings predictive of clinical behavior were not identified.