Towards a WHO classification of genetic tumour syndromes.

Almost 70 years ago, the World Health Organization (WHO) decided to propose a "Classification of Tumours". Since then, a systematic and extensive classification system for tumours has been continuously developed in successive editions and nowadays closely interlinks with coding systems for cancer registries like the International Classification of Diseases for Oncology (ICD-O). Whereas past editions had their focus on histopathological aspects of tumour classification in different organ systems and topologies, to which (somatic) genetic alterations increasingly contributed, the current fifth edition of the WHO Classification for the first time includes a separate "Blue Book" volume on "Genetic Tumour Syndromes". Along with chapters dedicated to tumour predisposition inferred by constitutional (germline) genetic pathogenic variants in the different organ-specific volumes of the classification, this new addition to the WHO classification highlights the increasing importance of constitutional genetic alterations for the diagnosis and clinical management of patients with such tumours. The WHO classification of Genetic Tumour Syndromes applies a hierarchical system based on four levels: the major (cellular) mechanism affected, the molecular pathway involved, the (clinical) syndrome, and the specific gene(s) affected. It provides - in part novel or modified - names to the genetic tumour syndromes as well as definitions and descriptions of clinical, epidemiologic, etiologic, pathogenetic and pathological aspects. Essential and desirable diagnostic criteria are given as well as rules for reporting, thus paving the way to international standardization. While the final version of the WHO Classification of Genetic Tumour Syndromes is in proof-stage, the present article, which is based on its beta-version, aims to provide an overview of the concepts underpinning the classification.