The International Classification of Diseases for Oncology, 4th Edition (ICD-O-4): An overview

The International Classification of Diseases for Oncology (ICD-O) was specifically developed for coding tumours according to their site of origin (topography), microscopic appearance or histology, behaviour and grade (morphology), and is optimal for use in cancer registries. The fourth edition of the ICD-O (ICD-O-4) aims to provide an improved structure of unique codes to existent and newly defined tumour entities and has been harmonised with the International Classification of Diseases 11th Edition (ICD-11).

Based on an International Association of Cancer Registries (IACR) survey of cancer registries, over 90 % of the respondents (250 of 276) agreed to an update of ICD-O-3.2 morphology by the addition of a fifth digit to the existing four-digit histology code. Following the 5th Edition of the WHO Classification of Tumours (WCT), a beta version of ICD-O-4 was developed and disseminated for open consultation by the International Agency on Research for Cancer (IARC) on the WCT website.

Following closure of the consultation period, the ICD-O-4 codes were finalized. The main changes in comparison to ICD-O-3.2 include the addition of a 5th alphanumeric digit to the histology code, changes in the first four digits of histology codes, changes of behaviour codes (e.g. pituitary adenoma code changed from /1 to /3), a new topography code for gastroesophageal junction (C16.7), detailed codes for extrahepatic bile ducts (C24.2, C24.3) and cystic duct (C24.4), change of the code for anal skin cancer from skin to anus (C44.5 to C21.3) and an optional additional digit in the topography code.

ICD-O-4 is compiled in consultation with pathologists, epidemiologists, public health researchers as well as the cancer registry community. The five-digit histology codes enable a hierarchical and detailed coding of tumours, while the new topography and behaviour codes reflect the evidence base on tumour aetiology, stage and behaviour.