Squamous Cells in the Thyroid Gland's Cytopathology and Histopathology: Institutional Experience.

Introduction Squamous cells are a rare finding in the thyroid gland and have been reported in a wide range of entities, from benign cysts to carcinomas. This study presents the diagnostic outcomes and potential pitfalls associated with squamous cell lesions in the thyroid gland, based on the experience of a tertiary centre's pathology laboratory. The main focus of the present study was on morphology, cytohistological correlation, immunohistochemistry and diagnostic pitfalls. Methods A pathology laboratory's information system was searched for squamous cell findings in the thyroid gland. Cytological samples were classified according to the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC), and histological specimens were categorised according to the World Health Organization (WHO) classification. Available microscopy slides were re-examined, and selected immunohistochemical and morphological features were documented. Results Our series included 42 thyroid gland specimens containing squamous cells, comprising 22 cytological samples and 20 histological samples. The most common diagnosis by the Bethesda Category for Thyroid Cytopathology (TBSRTC) was malignant (VI, 50%). According to histologies, most cases were malignant (n = 10, 62.5%), with anaplastic thyroid carcinoma (ATC) being the most frequent diagnosis (n = 5, 31.3%). There was one discordant case in which cytology suggested a benign lesion, whereas histology revealed an ATC. Morphologically, squamous cells in TBSRTC categories V and VI were atypical, and inflammatory infiltrates were more profound. CK5/6 and p40 were positive in all tested squamous cells. Conclusion The presence of squamous cells in thyroid samples can make diagnosis challenging, highlighting the need to correlate findings with clinical information. In this study, disagreements regarding malignancy between cytology and histology occurred in only one of nine cases. Only diagnostic discordance occurred in two out of nine cases.