Insular carcinoma of the thyroid with uncommon cytologic features: anisokaryotic cells and microfollicles containing dense colloid

Insular carcinoma of the thyroid is a rare neoplasm, constituting less than 5% of all thyroid tumors. It was Carcangiu et al. who first described this tumor, which exhibits an intermediate biologic behavior between well-differentiated and undifferentiated follicular carcinomas, as a distinct clinicopathologic entity. A 63-year-old female patient with thyroid enlargement was admitted to our institution. Thyroid ultrasonography revealed a 5x4x3cm solid nodule within the right thyroid lobe. The fine needle aspiration was highly cellular; there were individual cells with naked nuclei, loose aggregates, cohesive clusters of follicular cells and infrequent microfollicles with round-oval nuclei containing finely granular chromatin, and scant cytoplasm. There were two uncommon findings not previously reported in the literature. The first one is anisokaryotic nuclei, and the second one is the presence of dense colloid in the center of microfollicles. The aspiration biopsy was reported as malignant. The patient underwent bilateral total thyroidectomy. Histopathologically, the lesion was diagnosed as insular carcinoma. We believe that in addition to the previously described cytopathologic findings, microfollicles with dense colloid core and anisokaryosis may be indicators of insular carcinoma in thyroid FNACs.