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International Classification of Diseases for Oncology (ICD-O)


A multi-axial classification of the site, morphology, behavior, 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 tumors) for all types of tumors, thereby providing greater site detail for non-malignant tumors than is provided in ICD-10. In contrast to ICD-10, the ICD-O includes topography for sites of hematopoietic and reticuloendothelial tumors.

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

A separate one-digit code is also provided for histologic grading (differentiation).


Used principally in tumor or cancer registries for coding the site (topography) and the histology (morphology) of neoplasms, usually obtained from a pathology report.

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