A system for classifying cancers diagnosed in adolescents and young adults.

Background: Cancers in adolescents and young adults (AYAs) form a distribution distinct from those in children and older adults. As the increasing interest in AYA oncology has developed, the current classification system of neoplasms in AYAs has become outdated. Based on emerging knowledge in cancer biology and clinical advances, a new classification is proposed. Methods: The new system is based on morphologies and primary sites in the population aged 15 to 39 years. It accommodates information in WHO Blue Books on numerous groups of neoplasms and the recent update of the International Classification of Diseases for Oncology. Data from the Surveillance, Epidemiology and End Results (SEER) program of the US National Cancer Institute illustrate the proposed classification. Findings: The distinct distribution of cancers in AYAs reflects the inclusion of benign/borderline tumors of the CNS, in situ lesions for various carcinomas and neuroendocrine tumors in several locations. Diseases with striking increases in incidence, peculiar to AYAs in the SEER dataset, notably neuroendocrine tumors of the appendix and a re-named variant of carcinoma of the thyroid, are highlighted. New categories have been created to address Blood and Lymphatic Vessel Tumors and Nerve Sheath Tumors. Less than 0·5% of all cancers were designated as Unspecified Malignant Neoplasms Except CNS and fewer than 0·001% were not classifiable in the SEER data. Interpretation: The proposed classification system addresses the choice of the age range 15 to 39 years for AYAs and affords contrast with the previous classification as well as cancers in children and older adults. Characteristics which distinguish neoplasia in AYAs are emphasized. The ultimate goal of the new system is to improve cancer registration, diagnosis, treatment and health-related outcomes in this age group through standardized groupings of primary site and histology among different populations and over time. Funding Statement: The authors state: "No external funding." Declaration of Interests: The authors have no conflicts of interests to declare. Ethics Approval Statement: Not required.