The follicular lesions
The follicular lesions include follicular adenoma and follicular carcinoma. Papillary carcinoma also derives from follicular cells, but it is distinguished from the other lesions by its characteristic nuclear features. While for some conditions, such as Hashimoto`s thyroiditis and papillary carcinoma, FNA can be diagnostic, for the follicular lesions it represents a screening procedure. In fact, the diagnosis of these conditions cannot be made from a cytologic specimen alone, needing hystologic criteria (such as capsular infiltration or vascular invasion). The morphological distinction of hyperplastic adenomatous nodules, well-differentiated follicular carcinomas, and follicular variants of papillary carcinoma is difficult, even for cytologists with extensive experience of thyroid fine needle aspiration. Attempts to improve the preoperative diagnosis of thyroid nodules by use of strict instructions for obtaining adequate specimens and inclusion of clinical characteristics (such as sex, dimension of the nodule, features of the gland by palpation) have been reported.
FNA can be useful to identify follicular lesions which are suspicious for malignancy. The architecture is what differentiates a 'cytologically benign' from a 'cytologically suspicious' follicular nodule. Follicular carcinomas are rarely macrofollicular: they are usually composed predominantly of microfollicles and trabeculae or cords of crowded cells. The presence of these architectural patterns makes the nodule be diagnosed as cytologically suspicious. The cellularity, the size of cells and the amount of colloid support in the evaluation of the specimen; predominantly macrofollicolar lesions are very unlikely to be a carcinoma.
