Thyroid aspiration cytology: a "cell pattern" approach to interpretation

The key to the interpretation of thyroid fine needle aspiration is largely dependent on the recognition of various morphologic patterns of epithelial cells, usually follicular cells, and background elements, such as colloid. These morphologic patterns consist of 3 parts: 1) The arrangement of cells with respect to one another, 2) The cytologic features of individual cells, and 3) The presence of background elements. The cellular arrangements generally encountered in fine needle aspiration of the thyroid include the follicular patterns (macro-/normo-follicular and micro-follicular), the papillary pattern, the syncytial pattern, the dispersed cell pattern, and the cystic pattern. This article approaches some of the differential diagnostic challenges encountered while interpreting thyroid aspiration cytology by focusing first on the overall cellular arrangement to generate a differential diagnosis and then narrowing that differential by assessing cellular features of individual cells and the presence of background elements.