Background

The taxonomy of cancer provided by the WHO Classification of Tumours (WCT) is essential to cancer diagnosis, treatment and research. However, previous editions had significant gaps.  Meeting these needs systematically in a database though an editorial board and an evidence-based approach has been transformative. 

Objective

To present the major clinical impacts of the WCT, 5th edition. 

Methods

Review of the 5th edition and articles citing it.

Results

It is arguable that the largest direct clinical impact of the WCT on cancer incidence will be on cervical cancer.   For the elimination campaign, a new classification was required. This underpinned the subsequent IARC Handbook, which in turn provided the evidence to back up the guidance to countries issued by WHO.  The wording of “HPV-associated” and “HPV-independent” cancer is deliberate: it is less easy for computers and humans to confuse these words.  It should be noted that only the incidence of HPV-associated cervical cancers will be reduced by vaccination and enhanced surveillance. 

The decision to have a separate volume of the classification dedicated to paediatric cancer was made in 2017. It was initially intended to have this and the related genetic tumour syndromes volumes follow the organ-specific volumes at the end of the series.  However, as this became a major part of the IARC response to the WHA declaration on paediatric cancer (2020), it was brought forward, and published online in 2022, following an enormous effort on by many world experts at the height of the COVID19 pandemic. 

The final volume of the 5th edition on genetic tumour syndromes is of considerable scientific importance.  For the first time, it organises these syndromes according to the molecular pathways affected. It is probable that more than 10% of cancers are related to genetic tumour syndromes and the classification will underpin the systematic approach to research, as well as advances in patient and family management.

Conclusion

The process of putting together the 5th edition made it clear that there were gaps in knowledge that the WCT could readily identify, and that high quality evidence was needed to support editorial board decisions.  The formation of the International Collaboration for Cancer Classification and Research (IC3R) was designed to plug some of these gaps, while improving the flow of new information into the classification.  There have been a number of important outcomes, but one of the major ones, funded by the EU Evidence Gap Map project, is the definition of levels of evidence for studies of disease (pathology), which has been rapidly adopted by the EQUATOR network, the International Collaboration for Cancer Reporting (ICCR) and the Royal College of Pathologists. 

Classifications evolve on the basis of new knowledge, and this is now an increasingly well-managed process, overseen by a multidisciplinary editorial board composed of general and subject-specific experts with some 2,500 contributors, worldwide.  It forms one of the most important aspects of IARC’s work and is used daily around the world by those involved in cancer research, prevention, diagnosis and treatment.