Metastatic Papillary Thyroid Carcinoma in Pleural Effusion: Diagnostic Challenges in Effusion Cytology

Metastatic papillary thyroid carcinoma (PTC) presenting in serous effusion cytology is rare and usually reflects advanced disease with an adverse prognosis. Accurate identification in effusion specimens is diagnostically challenging because of overlapping cytomorphological features with other papillary malignancies. We report a case of a 74-year-old woman with a prior history of PTC who presented with bilateral pleural effusion. Cytological examination of the pleural fluid revealed papillary clusters of tumour cells, psammoma bodies and frequent intranuclear cytoplasmic inclusions. Cell block sections demonstrated tumour cell clusters that were immunoreactive for PAX8 and TTF-1. Subsequent pleural biopsy confirmed metastatic PTC. This case highlights the importance of careful cytomorphological assessment, correlation with clinical history and the critical role of immunocytochemistry-including PAX8, TTF-1 and thyroglobulin-in establishing the correct diagnosis of metastatic PTC in effusion cytology.