Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Nongynecologic Cytopathology Acta Cytologica 2013;57:177–183 DOI: 10.1159/000345696 Malignant Pleural Effusion Resulting from Metastasis of Thyroid Primaries: A Cytomorphological Analysis Matthew T. Olson Ayşe Nuransoy Syed Z. Ali Department of Pathology, The Johns Hopkins Hospital, Baltimore, Md., USA Introduction The most common primary tumors leading to pleural effusions are mesotheliomas, and tumors of the lung, breast, ovary, and the upper gastrointestinal tract [1–3]. Metastatic carcinoma from the thyroid to the pleural space is a much less common occurrence. The largest se- ries to date [4] consists of 10 cases and calculated the in- cidence of malignant effusion due to metastatic papillary thyroid carcinoma (PTC) to be 0.6%, and only 3 other case reports have demonstrated this disease manifesta- tion in individual patients [5–7]. The cytomorphological features have only rarely been described in a case report [7]. Thus, metastatic thyroid carcinoma is often not con- sidered in the differential diagnosis in the evaluation of malignant effusions. The increasing rate of thyroid cancer diagnoses and surgery is due to multiple factors that may [8] or may not necessarily reflect the increase in carcinogenic risk fac- tors. Nevertheless, the evaluation of malignant pleural ef- fusions from patients with a remote history of thyroid cancer will almost surely increase. This series demon- strates the clinical and cytomorphological features of ma- lignant pleural effusions due to metastatic thyroid cancer. To our knowledge, the cases presented here comprise the second largest series of this disease presentation to date and the first to focus on cytomorphological features. Key Words Anaplastic carcinoma  Papillary features  Pleural effusions  Primary cancer  Malignant effusions  Thyroid carcinomas Abstract Background: Malignant serous cavity effusion caused by primary thyroid cancer is extremely rare in routine clinical practice. Therefore, it is often not included in the differential diagnostic workup of patients presenting with positive effu- sion cytology. Methods: The clinical features were reviewed for 6 patients seen at our institution over the last 26 years for malignant effusion resulting from metastatic thyroid cancer. The cytomorphology from 4 of these cases was also re- viewed. Results: All of the patients found in this study pre- sented with malignant pleural effusion – other serous cavity effusions resulting from metastatic thyroid carcinoma were not seen. These comprised 0.25% of all patients with a known history of thyroid carcinoma and 0.67% of all malignant pleu- ral effusions. One patient had a history of bone metastases, but all the others had no pathological evidence of distant metastatic disease prior to the pleural effusion. Conclu- sions: Malignant pleural effusion rarely results from a thy- roid carcinoma after some latency. The diagnosis requires immunohistochemical staining as well as the pertinent clin- ical context. Copyright © 2013 S. Karger AG, Basel Received: August 23, 2012 Accepted after revision: November 7, 2012 Published online: February 28, 2013 Correspondence to: Dr. Syed Z. Ali Department of Pathology, Johns Hopkins Hospital, Path 406 600 North Wolfe Street Baltimore, MD 21287 (USA) E-Mail sali  @  jhmi.edu © 2013 S. Karger AG, Basel 0001–5547/13/0572–0177$38.00/0 Accessible online at: www.karger.com/acy