Dual malignancy in a thyroid; papillary thyroid carcinoma and small lymphocytic lymphoma; a report of a case with a cyto-histologic correlation.

Papillary thyroid carcinoma (PTC) is the most common thyroid malignancy. Simultaneous involvement of the thyroid gland by multiple malignancies, is a rare occurrence. Similarly, primary thyroid lymphomas are also rare. We are reporting a rare case of a dual thyroid malignancy; PTC with secondary thyroid involvement by chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), diagnosed on cytology and confirmed with flow cytometry, histology and immunohistochemistry. Imaging showed two hypermetabolic nodules, one in left parotid gland, and the other in the thyroid isthmus. Cytology smears showed features of PTC along with an atypical lymphocytic infiltrate, which co-expressed CD5, CD19, and CD23 on flowcytometry analysis. Subsequent thyroidectomy showed the atypical lymphocytic infiltrate involving the PTC papillae themselves. The diagnosis of dual malignancies on thyroid FNA is extremely rare and often unexpected, but is possible using appropriate ancillary studies.