Follicular thyroid carcinoma with signet ring cell morphology: fine-needle aspiration cytology, histopathology, and immunohistochemistry.

Thyroid follicular neoplasms with signet ring cell morphology represent a challenging cytological and histopathologic diagnosis. The low frequency of these neoplasms and their broad differential diagnosis contribute to this difficult scenario. Here, we present an exceptionally rare case of thyroid follicular carcinoma with signet ring cell morphology in a 62-year-old female. We analyze the characteristics in fine-needle aspiration cytology, histopathology, and immunohistochemistry and compare our results in a discussion with previous literature reports.