Invasive cervical thymoma masquerading as a solitary thyroid nodule. Report of a case studied by fine needle aspiration.

A 49-year-old woman underwent fine needle aspiration (FNA) biopsy of a presumed thyroid nodule. The initial cytopathologic interpretation suggested a chronic lymphocytic thyroiditis or a malignant lymphoma. The examination of frozen sections during surgery also suggested the presence of a lymphoma. However, histopathologic examination of permanent sections showed the lesion to be an invasive ectopic lymphocyte-predominant thymoma adjacent to the thyroid. Immunoperoxidase staining of FNA cell block sections and permanent sections showed positivity for keratin, proving the epithelial nature of the elongated and spindle-shaped tumor cells. This case high-lights the need to be aware of unusual lesions that may occur in the area of the thyroid; recognizing the potential diversity of "thyroid" masses that ultimately prove to be of nonthyroid origin should aid in making the correct cytologic differential diagnosis and interpretation of FNA samples obtained from such masses.