Evolving concepts in thyroid cytology

Concepts in thyroid diagnostics are evolving. As cytopathologists, we benefit from understanding the changes taking place in cytopathology practice as well as intersecting areas that may have implications for us. In this review, we discuss recent changes to 1. Classification systems, 2. Ancillary molecular testing modalities, and 3. Key metrics that affect thyroid cytopathology. The recent World Health Organization, Bethesda Thyroid Cytopathology, and American Joint Committee on Cancer classification systems have aspects that are designed to harmonize the clinical, cytopathologic, histomorphologic, and molecular findings for improved communication and patient management. New terminologies such as thyroid follicular nodular disease and low-risk follicular cell-derived thyroid neoplasms are introduced to reflect the subtle biologic nuances involving benign non-neoplastic and low-grade neoplastic conditions. The Bethesda Thyroid Cytopathology System has simplified its terminology, updated risk of malignancy estimates, and expanded the discussions on molecular testing, clinical and imaging assessments, and pediatric cytopathology. There is now a single term for each of the 6 diagnostic categories. The American Joint Committee on Cancer has refined the staging criteria to provide improved stratification of patient prognostication. Molecular testing using next-generation technology now offers large panels of markers that are sensitive for detecting the wide range of thyroid neoplasms. These panels were developed in North America and whether other regions of the world will choose similar tests remain to be seen. Finally, metrics such as molecular-derived risk of malignancy and molecular risk group may be viewed as surrogates of resection information and used to complement diagnostics, management, and quality assurance.