RESUMO
Papillary thyroid carcinoma (PTC) is by far the most common thyroid malignancy (over 85%) of all the thyroid cancers. It has excellent prognosis and 10-year survival rate in most of the cases (95%). Most of the tumors are indolent and do not recur or metastasize after removal. However, widespread metastases to lung, skeleton, central nervous system and, occasionally, other organs may be observed. In rare instances, this disease may metastasize to the pleura and manifest as a malignant pleural effusion (MPE) and portend poor prognosis. This article reports the cytomorphologic and immunocytochemical findings of a female patient with a symptomatic pleural effusion resulting from PTC metastatic to the pleura. Pleural fluid cytology revealed abundant papillary clusters with relatively nuclear pleomorphism, intranuclear cytoplasmic inclusions and nuclear grooves, small and distinct nucleoli as well as small discrete vacuoles. Psammoma bodies were not seen. Immunocytochemical staining was positive for TGB, EMA, Ber-EP4, CK19, and negative for TTF-1. Metastasis of PTC to pleural fluid is extremely rare and diagnosing the disease by cytology is challenging and requires medical expertise as well as knowledge of clinical context and immunocytochemical staining. Additionally, a cytologic diagnosis of MPE due to PTC provides important treatment information and plays an important role in prognosis.