Fine needle aspiration cytologic characteristics of hyalinizing trabecular adenoma of the thyroid

The cytologic characteristics of a histologically proven hyalinizing trabecular adenoma are presented. The solitary thyroid nodule was diagnosed cytologically as benign and histologically as malignant. Cytologically, the fine needle aspirate showed increased cellularity, with mild nuclear atypia and eosinophilic nuclear pseudoinclusions. Collections of an amorphous colloidlike material were noted surrounded by follicular cells. Inflammatory cells in the background suggested the possibility of a thyroiditis. Since the patient's antibodies were negative, a hemithyroidectomy was performed to rule out a neoplasm. The frozen section was interpreted as suggestive of medullary carcinoma since the tumor showed a poorly differentiated pattern of cells with marked nuclear atypia and abundant fibrous stroma (thought to be amyloid) separating the cells. The results of immunopathology and electron microscopy ruled out a medullary carcinoma. Upon review of the histology at a later date, the case was recategorized as a hyalinizing trabecular adenoma, proving the negative cytologic diagnosis to be correct.