ICD-O-4

International Classification of Diseases, 4th Edition (ICD-O-4)

Background and rationale

As new tumour types defined in the WHO Classification of Tumours series (also known as the WHO Blue Books) are increasingly adopted in pathology reporting and clinical practice, related classifications need to be updated accordingly. In discussions between WHO and IARC, and after the results of an International Association of Cancer Registries (IACR) survey of cancer registries, it was decided that the fifth edition of the WHO Blue Books would be followed by a new (fourth) edition of the International Classification of Diseases for Oncology (ICD-O-4).

Most notably, ICD-O-4 has a new morphology structure, with five-digit morphology codes, in which the fifth digit is either 0 or another alphanumeric value for more-specific coding. This approach was chosen to enable continuity and consistency with previous editions while greatly increasing the number of unique codes that can be assigned.

Alongside the updated ICD-O-4 morphology and topography listings, descriptions of changes between ICD-O-3.2 and ICD-O-4 are provided as reference material for cancer registries.