Addendum to version 9 of the American Joint Committee on Cancer (AJCC) neuroendocrine tumor cancer staging system. The content of the new version 9 is presented in a simple, easy-to-use format and includes the World Health Organization (WHO) tumor classification codes. Clinicians, registrars, researchers and other content users will benefit from the new content, including tables, descriptions and figures. Version 9 of the Cancer Classification System Neuroendocrine Tumor Supplement will be effective January 1, 2024 and will replace the 8th edition of the Cancer Classification System Neuroendocrine Tumor Supplement.