Fine needle aspiration cytology of insular carcinoma of thyroid

Poorly differentiated (insular) thyroid carcinoma is defined as a thyroglobulin-producing non-follicular non-papillary thyroid carcinoma, having an intermediate behavior between well-differentiated and anaplastic carcinomas. FNAC is widely used as aid for workup of thyroid gland lesion. However, scant information is available in the literature about cytologic findings of this rare entity. Ten cases of surgically resected insular carcinoma with a corresponding cytology were selected. The cytologic smears and histological sections were reviewed for presence of cytomorphologic features including cellularity, predominant cytoarchitectural pattern, additional cytologic co-patters pattern, cell size, cell shape, nuclear pleomorphism, nuclear/cytoplasmic (N/C) ratio, chromatin pattern, amount of cytoplasm, mitotic figures, colloid, background debris, nuclear grooves, and intranuclear pseudoinclusions. In all the cases, the cells were arranged predominantly in solid clusters. Focal microfollicular pattern was identified in five cases of which three cases showed presence of inspissated colloid within the follicles. Singly scattered malignant cells and bare nuclei were seen in all cases. Cells were monomorphic, round with high N/C ratio, finely granular chromatin and inconspicuous nucleoli. Background showed presence of cellular debris in two cases. Mitotic figures were obvious and atypical mitosis was also identified. Cellular smears composed of monomorphic population of small cells arranged in large clusters and sheets with high N/C ratio and high mitosis suggest the possibility of insular carcinoma. Background cellular debris/necrosis also supports the diagnosis. Cell block preparation in these cases may be of additional help in accurate diagnosis.