Update from the 4th Edition of the World Health Organization Classification of Head and Neck Tumours: What Is New in the 2017 WHO Blue Book for Tumors and Tumor-Like Lesions of the Neck and Lymph Nodes.

The World Health Organization (WHO) 2017 Classification of Head and Neck Tumors ("Blue Book") will now include a new chapter on tumors and tumor-like lesions of the neck and lymph nodes, which was not included in the previous edition. Tumors and tumor-like lesions, including a variety of cysts and metastases, can arise in any component in the neck, including soft tissue, lymph nodes, and developmental remnants. The pathology and clinical features of metastatic carcinoma of unknown primary in the head and neck has changed dramatically in the last several years. Many of these tumors which were previously diagnosed as unknown primary are now identified as oropharyngeal and nasopharyngeal carcinomas related to human papillomavirus (HPV), less commonly to Epstein-Barr virus (EBV) and occasionally even to Merkel cell polyomavirus. Many unusual features can arise in these metastases, such as undifferentiated morphology, extensive cystic change with central degeneration, gland formation, and even ciliated cells. Rarely, carcinoma in the neck can arise in association with a heterotopic tissue, primarily thyroid or salivary gland tissue. Tumor-like lesions include branchial cleft cysts, thyroglossal duct cyst, dermoid and teratoid cyst, and ranula. Pathologists should be familiar with the diagnostic features and clinicopathologic corrections of these neck lesions in order to correctly diagnosis them and to provide for proper clinical management. This article will briefly describe the pathologic and clinical features of these entities as they are covered in the new 2017 Blue Book.