The main idea is that cancer registries need a coding system that records both where a tumor started and what its microscopic appearance is, in a single, standardized framework. ICD-O-3, the International Classification of Diseases for Oncology, third edition, is built exactly for that purpose. It provides separate codes for the tumor site (topography) and for morphology (histology), and it includes a behavior component to indicate whether the tumor is malignant, in situ, or of uncertain behavior. This two-part structure lets registries consistently capture incidence data across different cancers, enabling reliable comparisons and research. For example, a breast cancer with ductal carcinoma histology would be coded with the breast site code and a morphology code that specifies ductal carcinoma and its malignant behavior. Other options are broader clinical coding or diagnostic classification systems that aren’t designed specifically to capture the site and microscopic type of cancers in registry data. SNOMED is a comprehensive clinical terminology, while ICD-9-CM and ICD-10-CM are mainly used for general diagnoses and billing, not the dedicated site-and-m morphology framework registries rely on. Therefore, ICD-O-3 is the correct standard for this purpose.