Cytodiagnosis of Primary Thyroid Lymphoma Coincident with Unnoticed Papillary Thyroid Carcinoma: A Case Report and Review of the Literature.

Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer, whereas primary thyroid lymphoma is very rare. Here, we report a case in which a right-sided nodule measuring 4.3 × 2.2 cm was examined using fine-needle aspiration biopsy. This revealed abundant monomorphic non-cohesive large lymphoid cells without thyroid follicular cells, on which basis acytodiagnosis of lymphoma coincident with lymphocytic thyroiditis was made. Subsequent histologic examination revealed CD45-, CD20+, and Bcl-6 + and cytokeratin-, CD3-, CD5-, and CD30-negative tumor cells arranged diffusely in the whole thyroid coexisting with a separate PTC nodule sized 1.3 × 1.0 cm in the right lobe. The key point exemplified by this case is that a cytodiagnosis of this extremely rare coexistence of PTC and lymphoma can be made by adequate sampling of both nodules preoperatively. In our case, only one nodule formation was sampled, and therefore the coexisting PTC was not detected with cytology preoperatively.