Since it was first published in 1976, the International Classification of Diseases for Oncology (ICD-O) has been internationally recognized as the definitive classification of neoplasms. It is used by cancer registries throughout the world to record the incidence of malignancy and survival rates, and the data produced are used to inform cancer control, research activity, treatment planning, and health economics.

The classification of neoplasms used in ICD-O links closely to the definitions of neoplasms used in the WHO/IARC Classification of Tumours series which are compiled by consensus groups of international experts and, as such, the classification is underpinned by the highest level of scientific evidence and opinion.

ICD-O consists of two axes (or coding systems), which together describe the tumor:

  • the topographical code, which describes the anatomical site of origin (or organ system) of the tumor, and
  • the morphological code, which describes the cell type (or histology) of the tumor, together with the behavior (malignant or benign).

The third edition of ICD-O (ICD-O-3) has been available in printed format since 2000. In September 2011, following approval by the WHO/IARC Committee for ICD-O-3, the classification was updated with a number of new or modified codes and terms (ICD-O-3 First Revision, or ICD-O-3.1).

ICD-O-3 online now makes available ICD-O-3 and ICD-O-3.1 as fully searchable electronic resources. The alphabetical index of the printed book has been replaced with an efficient search tool to enable the user to quickly identify specific entities. In addition, the online format has enabled WHO/IARC to enhance the listings with useful definitions of entities from the WHO/IARC Classification of Tumours series and other sources.

The International Classification of Diseases for Oncology (ICD-O) is a dual classification, with coding systems for both topography and morphology.

The topography code describes the anatomical site of origin of the neoplasm and, while it uses the same categories as in the neoplasm section of Chapter II of the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10), some of the individual codes are different. The code always has a prefix of “C”, followed by a three-digit number that indicates the site (two digits) and the subsite (one digit), separated by a decimal point. For example, in C18.4, the C18 indicates that the site is the colon and the 4 indicates that the subsite is the transverse colon.

The morphology code describes the characteristics of the tumor itself, including its cell type and biological activity. The code is composed of four digits that indicate the cell type or histology and one digit that indicates the behavior. The first four digits are separated from the last (behaviour) digit by a forward slash (/). The behavior digit can be 0 (benign), 1 (uncertain behavior), 2 (carcinoma in situ), 3 (malignant, primary site), 6 (malignant, metastatic site), or 9 (malignant, uncertain whether primary or metastatic site).

The Morphological codes tab at the top of the page first provides an option to view all the codes listed in the original ICD-O-3 publication (2000), all the codes listed in the first revision of ICD-O-3 (2011), or all the codes added, changed, or revised in the first revision. Selecting one of these options will then provide a listing in numerical code order, from 8000/0 to 9992/3.

1 International Classification of Diseases for Oncology, Third Edition, First Revision. Geneva: World Health Organization, 2013.