Fine Needle Aspiration Cytology in Unveiling a Rare Diagnosis of Lacrimal Sac Solitary Fibrous Tumour and Its Cytological Attributes

Solitary fibrous tumour (SFT) of the lacrimal sac is an uncommon lesion that may present as a slow-growing, painless medial canthal swelling, and its cytological features are not widely characterised. We report the case of a 48-year-old male patient who presented with a firm, non-tender 4 × 4 cm mass at the left medial canthus with gradual progression over several months. Fine-needle aspiration smears were moderately cellular and showed spindle to oval cells arranged in loose clusters, singly scattered, and focally adherent to endothelial fragments. The cells exhibited pale wispy cytoplasm with elongated processes, round to oval nuclei with fine chromatin and inconspicuous nucleoli, without necrosis or significant atypia. Immunocytochemistry performed on the destained smears demonstrated a diffuse cluster of differentiation (CD)34 positivity. Radiological evaluation revealed a well-circumscribed, enhancing lesion expanding the lacrimal sac and nasolacrimal duct. Complete excision of the lesion was undertaken. Histopathology showed a cellular spindle cell tumour arranged in short fascicles and whorls with a prominent branching vascular network. Immunohistochemistry confirmed the diagnosis with diffuse cytoplasmic CD34 and strong nuclear Signal Transducer and Activator of Transcription 6 (STAT6) expression. No significant mitotic activity or necrosis was identified.