Metastatic bronchioloalveolar carcinoma presenting as a solitary thyroid nodule: report of a case with fine-needle aspiration cytopathology

Although cancers metastatic to the thyroid are frequently mentioned in autopsy studies, such a finding is quite rare in routine clinical practice. Metastatic non-small-cell carcinomas to the thyroid may present a diagnostic dilemma, particularly when they share morphological similarities with primary thyroid tumors. Herein, we report a case of metastatic bronchioloalveolar carcinoma that presented as an isolated left thyroid nodule in a 68-year-old woman. The aspirates were cellular and showed numerous papillary-like tissue fragments, elongated nuclei with prominent nuclear grooves, frequent mitoses, and psammoma bodies. The latter features raised the possibility of papillary thyroid carcinoma. However, also seen were three-dimensional tumor nests and acinar-forming fragments. Immunostains (positive for cytokeratin-7 and carcinoembryonic antigen and negative for cytokeratin-20 and thyroglobulin) confirmed the metastatic nature of the carcinoma. In a patient with known primary neoplasm, the differential diagnosis of a thyroid nodule should always include a metastatic lesion along with primary neoplasia.