Cytopathology of insular carcinoma of the thyroid

BACKGROUND. Insular carcinoma of the thyroid (ICT) first was reported in 19 84. To the authors' knowledge, few cytology reports have been published sin ce that time. The authors describe the cytologic features of six tissue-pro ven ICTs and propose criteria that suggest its diagnosis. METHODS. Four cases were thyroid fine-needle aspiration (FNA) samples. Two cases were FNAs of metastases. All cases were found to be classic ICT on ex amination of primary or metastatic surgical specimens. RESULTS. Three cases originally were diagnosed as carcinoma, including two FNAs of metastatic sites and one thyroid FNA. Two additional thyroid FNAs w ere diagnosed as suspicious for malignancy, favor follicular neoplasm. One case was termed a neoplasm, favor follicular type. Smears showed high cellu larity and scanty colloid. Three cases were found to contain some microfoll icles. One case, showed a few papillae. Necrosis and mitosis were rare. Cel ls were round with pale, poorly defined cytoplasm. Nuclei were round and mo nomorphic with finely granular chromatin, mild hyperchromasia, smooth nucle ar membranes, and small nucleoli. Nuclear grooves and inclusions were rare. CONCLUSIONS. Three cases were diagnosed as suspicious for follicular neopla sm, the main differential diagnosis of ICT. Both tumors exhibited high cell ularity and scanty colloid. However, ICT showed a predominance of single ce lls whereas follicular neoplasms reveal microfollicles with more nuclear at ypia, There is cytologic overlap between these two neoplasms. Papillary thy roid carcinoma should be distinguished from ICT easily because the latter u sually does not reveal the classic cytologic features associated with the f ormer. ICT should be considered in the differential diagnosis of follicular neoplasms. Features favoring ICT are predominance of single cells, small l oose nests of cells with few microfollicles, and little nuclear atypia. (C) 1999 American Cancer Society.