Extensive spindling in medullary thyroid carcinoma posing a diagnostic challenge: A case report.

Medullary thyroid carcinoma is a neuroendocrine tumor derived from parafollicular or 'C' cells with sporadic and familial forms. Fine needle aspiration cytology is a quick way to establish the diagnosis. Sometimes a usual tumor may show prominent spindling on cytomorphology due to sampling issues, which poses a diagnostic challenge. When encountering spindled tumor cells on cytology smears from the thyroid, differential diagnoses encompass many benign and malignant lesions. We present cytological findings of the usual type of medullary thyroid carcinoma showing extensive spindling on cytology smears, which was a diagnostic dilemma. An accurate diagnosis was reached with the judicious use of immunocytochemistry on the cell block. We also discuss the available literature, differential diagnoses, and how to troubleshoot.