Solitary fibrous tumour: a diagnostic challenge for the cytopathologist

Solitary fibrous tumour (SFT) is an uncommon spindle cell tumour that can occur in a variety of locations. Cytological features of this tumour have only rarely been reported in the literature. We describe the cytomorphological features of SFT with an emphasis on diagnostic pitfalls.

We retrieved nine cases of histopathologically proven SFT. Three cases had sampling error with inadequate smears and, therefore, six cases with adequate cellularity were analysed for cytological findings. The cytomorphological features and the differential diagnoses on fine needle aspiration cytology (FNAC) are discussed.

No definitive cyto-diagnosis of any of these cases was possible because of the morphological overlap with various soft tissue tumours and other tumour types. There was one false-positive case, in which the possibility of sarcoma was suggested due to the presence of scattered atypical cells. Cytologically, the smears from the SFTs showed spindle to plump cells embedded in metachromatically staining dense ropy collagen material. The cells usually had oval to spindle shaped nuclei, bland chromatin and wavy elongated pale staining cytoplasm.

A diagnosis of SFT on cytology smears is challenging. Careful attention given to certain cytological features in an appropriate clinicoradiological setting and application of immunochemistry, including CD34 and CD99 immunostaining on cytological samples, can help in the diagnosis of SFT in some cases. It is important to consider cytological overlaps of this tumour in order to avoid false-negative or false-positive results.