System for the classification of tumors according their site (location) and their histology (morphology). Hosted by WHO (IARC) in the ICD-O (International classification of diseases (oncology)

Classification structure

A multi-axial classification of the site, morphology, behaviour, and grading of neoplasms.

The topography axis uses the ICD-10 classification of malignant neoplasms (except those categories which relate to secondary neoplasms and to specified morphological types of tumours) for all types of tumours, thereby providing greater site detail for non-malignant tumours than is provided in ICD-10. In contrast to ICD-10, the ICD-O includes topography for sites of haematopoietic and reticuloendothelial tumours.

The morphology axis provides five-digit codes ranging from 8000/0 to 9989/3. The first four digits indicate the specific histological term. The fifth digit after the slash (/) is the behaviour code, which indicates whether a tumour is malignant, benign, in situ, or uncertain (whether benign or malignant).

A separate one-digit code is also provided for histologic grading (differentiation) or cell type in lymphomas and leukemias.

A complete code consists of 10 digits (4 for topography, 6 for morphology, behavior and grading), separated by a "." after the 3rd digit in topography and a "/" after the 4th digit in morphology (8th digit in total). Qualifier for laterality (1 additional digit) may be given after the topography code.