Metastasis of unknown primary site in serous pleural effusion is common and cytopathological analysis may have important therapeutic implications. Primary lung and metastatic tumors may share similar cytopathological features with difficulties in differential diagnosis especially in routine cytology practice. TTF-1 is highly specific and sensitive in the diagnosis of certain types of primary lung carcinomas, especially adenocarcinoma and small cell carcinoma in histological sections. In this study, we determined the usefulness of TTF-1 in pleural fluids for the differential diagnosis of carcinomas of pulmonary and extrapulmonary origin.

Cytological materials from 36 patients with malign pleural effusion were evaluated. The diagnosis of primary site was identified for all cases by clinical, radiological, cytological and/or histological correlation. Twenty-five out of 36 cases were primary lung carcinomas and 11 were metastatic. Immunohistochemical study was performed on the cell block sections or unstained effusion cytology specimens. Among 25 metastatic pulmonary carcinomas, 19 cases showed strong nuclear positivity with TTF-1 (15 adenocarcinoma, 3 nonsmall cell carcinoma and 1 small cell carcinoma) (sensitivity %76) and 6 cases were negative (2 nonsmall cell carcinoma, 1 adenocarcinoma, 1 squamous cell carcinoma, 1 large cell carcinoma, 1 small cell carcinoma). None of the 11 metastatic extrapulmonary carcinomas stained for TTF-1 (6 breast, 2 gastric, 1 kidney, 1 ovary, 1 oesophagus) (specificity %100).

Our results show that, TTF-1 is sensitive and also highly specific marker in discriminating between metastatic pulmonary and extrapulmonary carcinomas in effusion cytology specimens.