Solid cell nests in Hashimoto's thyroiditis sharing features with papillary thyroid microcarcinoma

Solid cell nests (SCN) associated with Hashimoto's thyroiditis may show some atypical nuclear features including prominent nuclear grooves, enlarged overlapping nuclei and nuclear clearing. These features are sometimes mistaken for papillary thyroid microcarcinomas especially when the SCN are numerous. We reviewed SCN associated with Hashimoto's thyroiditis in 12 patients selected from 1,420 archival routinely processed formalin-fixed, paraffin-embedded thyroid specimens of Hashimoto's thyroiditis in which there was more than ten SCN per slide. In addition to the atypical nuclear features, there was a distinct eosinophilic basement membrane surrounding the SCN. Immunohistochemical analysis showed that the SCN were strongly positive for p63, stained weakly for TTF-1 and were negative for thyroglobulin, HBME-1, and calcitonin. This was compared to papillary thyroid microcarcinomas which were strongly positive for thyroglobulin, TTF-1, HBME-1, and variably positive for p63, while calcitonin and chromogranin were negative. These histological and immunophenotypic features can be used to distinguish SCN from papillary thyroid microcarcinomas associated with Hashimoto's thyroiditis.