Cytomorphological spectrum of solitary fibrous tumour: revisited

Background Solitary fibrous tumour (SFT) is a tumour of mesenchymal origin. Its diagnosis on cytology is challenging, owing to variation in cellularity, sparsely distributed cellular and stromal components. Cytomorphological findings for this type of tumour have rarely been described in the literature-only a few case reports and the occasional case series have been presented thus far. We present the cytomorphological features of SFT with special emphasis on immunochemical findings. Materials and Methods We present cytological data from eight cases of histopathologically proven SFTs. The cytomorphological features, immunochemical markers and differential diagnostic entities on fine needle aspiration cytology are discussed. Results Fine needle aspiration was performed at various anatomical sites. Cytology smears showed variable cellularity, with tumour cells arranged in loose clusters and as singly scattered cells. Interlacing fascicles with palisading of cells was noted. The cells were predominantly spindle to elongated, having moderate cytoplasm with elongated wavy nuclei. These nuclei had fine to coarse chromatin, with inconspicuous to prominent nucleoli. There was prominent, metachromatically staining, amorphous to fibrillary, collagenous to myxoid matrix material associated with the tumour cells. Other findings included intranuclear pseudo-inclusions, multinucleated giant cells and atypical mitoses. Cytological diagnoses offered varied from 'spindle cell neoplasm' to 'spindle cell sarcoma' or 'suggestive of sarcoma'. Immunocytochemistry (ICC) performed on cell block sections showed positivity for STAT6, CD34 and Bcl-2. Conclusion Cytological diagnosis of SFT can be challenging. A careful search for characteristic cytomorphological features is diagnostically helpful. The cytomorphology should be interpreted with caution, with an appropriate ICC panel, including STAT6 and CD34.