An overview of the new International Classification of Diseases for Oncology, 4th edition (ICD-O-4)

A new article from the International Agency for Research on Cancer (IARC) and partners summarizes the rationale and content of the new fourth edition of the International Classification of Diseases for Oncology (ICD-O). ICD-O is a long-standing system for coding tumours according to their site of origin (topography), microscopic appearance or histology, behaviour, and grade. It is optimal for use in cancer registries. The fourth edition (ICD-O-4) provides an improved structure of unique codes for existing and newly defined tumour entities and has been harmonized with the 11th revision of the International Classification of Diseases (ICD-11).

ICD-O-4 was developed after a review of the 5th edition of the World Health Organization (WHO) Classification of Tumours series, produced by IARC, which recognized several new tumour entities and introduced molecularly defined classifications for certain tumours with clinically relevant molecular profiles. Upon the completion of the 5th edition, there was a compelling need for an updated ICD-O that would allow for unique codes to be assigned to the newly defined tumour entities, facilitating the accurate coding of tumours included in the current 6th edition and future editions of the WHO Classification of Tumours. The finalized ICD-O-4 topography and morphology tables will be available on the WHO Classification of Tumours Online website, and the ICD-O-4 book, to be published by WHO, is in preparation.

Znaor A, Rous B, Levy GG, Watanabe R, Jakob R, Krpelanova E, et al.
The International Classification of Diseases for Oncology, 4th edition (ICD-O-4): an overview
Cancer Epidemiol. Published online 22 January 2026;
https://doi.org/10.1016/j.canep.2026.102989

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View the WHO Classification of Tumours Online website