Background
The WHO Classification of Tumours (WCT) underpins cancer diagnosis, research, and clinical management worldwide. Traditionally based on expert consensus, tumour classification has now adopted an evidence-based approach, with the need to adapt to the rapid growth and complexity of research evidence. Evidence-based pathology (EBP), which applies the principles of evidence-based medicine to the field of pathology, offers a framework and methodologies to ensure that classification decisions are informed by the best available research evidence. The development of the WHO Classification of Tumours  6th edition (WCT-6) marks a change towards systematically incorporating EBP approaches, by analysis of citations of the  WHO Classification of Tumours   5th edition (WCT-5) and analysis of currently available research evidence by the European Union funded ‘Mapping the Evidence for the WHO Classification of Tumours: a Living Evidence Gap Map by Tumour Type’ (WCT EVI MAP) project.
Objectives
To map the cited evidence base in the 5th edition WHO Classification of Tumours (CMs) and describe the currently available research evidence provided by the WCT EVI MAP project (EGMs) for the 6th edition.
Methods
The WCT EVI MAP project adapts evidence gap map methodology to synthesise the scientific literature published on topics relevant for cancer classification and produces living EGMs applying the WCT framework and its tumour types. These maps visualise evidence by tumour type and characteristics of each tumour classifying them into high, medium or low-level of evidence based on the study design. A similar methodology is applied to devleop citation maps CMs, synthesising the references cited in the WCT-5.
Results
The WCT EVI MAP has developed 56 EGMs to date for selected tumour types, highlighting areas of high-level of evidence and critical gaps requiring further research. To date, nine CMs (on breast, digestive, female genital tract, soft tissue and bone, thoracic, central nervous system, urinary, haematolymphoid, and head and neck tumours) have been developed.
Conclusions/Implications
The integration of WCT EVI MAP findings represents an important evolution in the development of WCT. They facilitate access, visualisation and analysis of the citations of the WCT-5 based on CMs, while giving access to the current research evidence in the field through EGMs, supporting more transparent, structured, and reproducible use of evidence by WCT6 editorial boards. These CMs and EGMs thus ensures integration of EBP practices into the WCT-6 editorial process. In addition, combined CMs and EGMs allow to assess the changing trends in cancer research, incorporating studies on molecular tumour characteristics. Regarding EGMs, a visual representation of the number of studies and their level of evidence permits to easily identify gaps in the published evidence and pockets of low-level evidence that require further research.  By moving towards a systematic evidence synthesis considering high-level of evidence and generating decision- making support tools, WCT-6 is developing a more transparent and consistent approach to an evidence based tumour classification.