Non-hyalinizing trabecular thyroid adenoma: A novel thyroid tumor with diagnostic pitfalls of hyalinizing trabecular adenoma and medullary thyroid carcinoma

Background: Only one thyroid follicular cell-derived tumor with a purely trabecular growth pattern has previously been described. This report aims to describe the histological, immunohistochemical, and molecular findings of our second case, propose a novel thyroid tumor, and discuss its diagnostic pitfalls. Case presentation A 68-year-old female presented with an encapsulated thyroid tumor composed of thin and long trabeculae. No papillary, follicular, solid, or insular patterns are observed. The tumor cells were elongated or fusiform and arranged perpendicular to the trabecular axis. No nuclear findings of papillary thyroid carcinoma and increased basement membrane material were found. Immunohistochemically, the tumor cells were positive for paired-box gene 8, thyroid transcription factor-1, and negative for thyroglobulin, calcitonin, and chromogranin A. Inter- and intra-trabecular accumulation of type IV collagen-positive materials was not demonstrated. No PAX8/GLIS1, PAX8/GLIS3, BRAF, HRAS, KRAS, NRAS, TERT, CTNB1, PTEN , or RET were detected. Conclusions We report our case as a novel disease entity called non-hyalinizing trabecular thyroid adenoma, which has the diagnostic pitfalls of hyalinizing trabecular tumor and medullary thyroid carcinoma.