Distinction between colloid nodules and follicular neoplasms of the thyroid. Further observations on cell blocks

The differentiation between colloid nodules and follicular neoplasms may be difficult in lesions yielding only microfollicles by fine needle aspiration (FNA). In a retrospective study of 35 follicular lesions, the FNA smears and cell blocks and the excised specimens were reviewed for possible distinguishing features. Columnar cells lining follicles and prominent sinusoidal stroma were seen only in follicular neoplasms; these were more easily appreciated in the cell blocks. Nuclear features and fibrosis were not helpful because they were found in both groups of lesions. Attention to the nuclear morphology was important, however, for the diagnosis of the follicular variant of papillary carcinoma. A repeat FNA might be helpful in obtaining hyperplastic papillae and fragments of dilated follicles for a diagnosis of colloid nodule in half of the cases. A few lesions with predominant microfollicular patterns were even problematic to diagnose on the excised specimens.