Changes in the WHO classification and new expertise in salivary gland tumor pathology

The WHO classification of salivary gland tumours has increased significantly from nine histological types of epithelial tumors in the first edition (1972) to 36 in the current fifth edition (2022). This is largely because of the global accumulation of case data, advances in immunohistochemistry, and developments in molecular analysis techniques. However, the important and frequent histological types (e.g., Pleomorphic adenoma, Warthin tumor, Basal cell adenoma/adenocarcinoma, Mucoepidermoid carcinoma, Adenoid cystic carcinoma, Acinic cell carcinoma, Salivary duct carcinoma and Epithelial-myoepithelial carcinoma) described in the second edition (1991) have not changed, and various differential diagnoses among them remain challenging in routine practice. 　Although the recent elucidation of tumorspecific gene mutations has opened the door to new molecular targeted therapies and brought about major changes in clinicians (i.e., antiandrogen agents, trastuzumab, TRK inhibitors), the first step in a pathological diagnosis is still to interpret detailed histological findings and perform appropriate immunostaining. This article describes the evolution of the WHO classifications and the main diagnostic points in the major tumor groups, and provides an overview of the new tumor entities adopted in the current classifications. Furthermore, I outlined the current status of new therapeutic strategies based on specific molecular pathological findings.