Supplementary Material for: Challenging Subtypes and Mimics of Papillary Thyroid Carcinoma: Highlights from a 22nd International Congress of Cytology Slide Seminar (Case Reports)

Recent advances in pathology have expanded our understanding of cytogenesis, morphologic features, molecular classification, and biological behavior of tumours. Consequently, histological classification of thyroid neoplasms has become more precise, making cytological diagnosis more challenging. The Focus on Cytomorphology with the Professor Slide Seminar at the 22nd International Congress of Cytology addressed this issue by presenting 4 cases, emphasizing subtle morphologic clues, use of ancillary testing, and the value of clinicopathologic correlation. Cribriform morular thyroid carcinoma (CMTC), previously categorized under papillary thyroid carcinoma (PTC) is now classified as tumour of uncertain histogenesis. Awareness of key cytological features, correlation with ultrasound findings, and supportive immunocytochemistry for beta-catenin and/or estrogen receptors led to the correct diagnosis. Hyalinizing trabecular Tumour (HTT) shares nuclear features of PTC and diagnosis on cytological specimens is challenging, potentially resulting in a false positive diagnosis. However, recognition of elongated/polygonal cells with peculiar intra-trabecular hyaline material is a clue to its diagnosis. Molecular studies demonstrating GLIS gene rearrangements (PAX8::GLIS3 and PAX8::GLIS1) are specific for HTT. High-Grade Differentiated Thyroid Carcinoma (HGDTC) retains the features of differentiated carcinoma and also shows high-grade features. However, the high-grade features may not be apparent on cytological specimens, making the presurgical cytological diagnosis very challenging. Furthermore, there is pathobiologic heterogeneity among the subtypes of HGDTC with the diffuse sclerosing subtype of HGDTC behaving less aggressively. Columnar cell carcinoma (CCC-PTC) is a rare subtype of PTC that is characterized by hypercellularity and pseudostratification. Co-expression of TTF-1 and CDX-2 is a useful clue to its diagnosis.