The International Classification of Diseases for Oncology (ICD-O) is a domain-specific extension of the International Statistical Classification of Diseases and Related Health Problems for tumor diseases. This classification is widely used by cancer registries. It is currently in its third revision (ICD-O-3). ICD-10 includes a list of morphology codes. They stem from ICD-O second edition (ICD-O-2) that was valid at the time of publication. Axes The classification has two axes: topography and morphology. Morphology The morphology axis addresses the microscopic structure (histology) of the tumor. This axis has particular importance because the Systematized Nomenclature of Medicine ("SNOMED") has adopted the ICD-O classification of morphology. SNOMED has been changing continuously, and several different versions of SNOMED are in use. Accordingly, mapping of ICD-O codes to SNOMED requires careful assessment of whether entities are indeed true matches. Topography The topography axis addresses the tumor's site in the body. It is standardized with the C section of ICD-10. There were no changes in the topography axis between ICD-O-2 and ICD-O-3. See List of ICD-10 codes#(C00–C97) Malignant Neoplasms for examples. International Classification of Diseases for Oncology, Third Edition (ICD-O-3) 5th Digit Behaviour Code for Neoplasms /0 Benign /1 Uncertain whether benign or malignant Borderline malignancy Low malignant potential Uncertain malignant potential /2 Carcinoma in situ Intraepithelial Noninfiltrating Noninvasive /3 Malignant, primary site /6 Malignant, metastatic site Malignant, secondary site /9 Malignant, uncertain whether primary or metastatic site Morphology Codes (ICD-O-3) 800 Neoplasms, NOS M8000/0 Neoplasm, benign Tumor, benign Unclassified tumor, benign M8000/1 Neoplasm, uncertain whether benign or malignant Neoplasm, NOS Tumor, NOS Unclassified tumor, uncertain whether benign or malignant Unclassified tumor, borderline malignancy M8000/3 Neoplasm, malignant Tumor, malignant, NOS Malignancy Cancer Unclassified tumor, malignant Blastoma, NOS M8000/6 Neoplasm, metastatic Neoplasm, metastatic Tumor, metastatic Tumor, secondary Tumor embolus M8000/9 Neoplasm, malignant, uncertain whether primary or metastatic Unclassified tumor, malignant, uncertain whether primary or metastatic M8001/0 Tumor cells, benign M8001/1 Tumor cells, uncertain whether benign or malignant Tumor cells, NOS M8001/3 Tumor cells, malignant M8002/3 Malignant tumor, small cell type M8003/3 Malignant tumor, giant cell type M8004/3 Malignant tumor, spindle cell type Malignant tumor, fusiform cell type M8005/0 Clear cell tumor, NOS M8005/3 Malignant tumor, clear cell type 801–804 Epithelial Neoplasms, NOS M8010/0 Epithelial tumor, benign M8010/2 Carcinoma in situ, NOS Intraepithelial carcinoma, NOS M8010/3 Carcinoma, NOS Epithelial tumor, malignant M8010/6 Carcinoma, metastatic, NOS Secondary carcinoma M8010/9 Carcinomatosis M8011/0 Epithelioma, benign M8011/3 Epithelioma, malignant Epithelioma, NOS