ICDO3 (International Classification of Diseases for Oncology, Third Edition), often spelled “ICD-O-3”, is an international double classification system for coding neoplastic diseases by specifying both their morphology and anatomical location. It is currently implemented as a standard vocabulary, with OMOP-generated mappings and hierarchical relations to concepts from SNOMED CT vocabulary and other relationships mimicking SNOMED CT's internal model.

It also features precoordinated set of combined concepts featuring both those that were validated by SEER and reported by vocabulary users from encounters in their source data.

There are three concept classes in ICDO3 vocabulary.

Concepts that represent various body sites.

ICDO Topography concepts correspond to malignant neoplastic Condition concepts in ICD10 vocabulary and use the same 4 character alphanumeric codes.

Benign neoplasms, in-situ neoplasms or neoplasms of uncertain behavior will still use same ICD10 codes for topography.

Concepts that represent various morphologies of neoplastic diseases. Histology codes are built from 2 parts separated by slash. First part consists of 4 digits and encodes morphology. Last digits encodes behavior of the neoplasm.

Some cancer registries may store behavior of the neoplasm in a separate field. In OMOP morphology and behavior are treated as a single combined entity.

Concepts that are formed by combining Histology and Topography concepts together to represent actual disease. Since number of all possible combinations is enormous and no universal validation list exists, only limited number of valid combinations is included.

Codes of ICDO conditions are formed by concatenating Histology and Topography codes over hyphen. Source data are not expected to have such codes: usually codes are provided separately.

Some ICDO Condition concepts feature only Histology or only Topography, as seen from their names and codes. These codes are created for each existing histology and topography to accommodate for incomplete source data.

All ICDO conditions have equivalence mappings or hierarchical relations built to SNOMED concepts from SNOMED Clinical Finding sub-hierarchy. This way ICDO3 Condition concepts form seamless extension of SNOMED hierarchy.

Relations for conditions are built automatically, using manual relations of Topographies and Histologies to corresponding SNOMED concepts. Existing mappings, list of combinations and logic for building the condition hierarchy can be found on projects github.

Since all concepts in ICDO3 vocabulary map to or form hierarchical relations with concepts in SNOMED CT, ICDO3 in OMOP can be seen as seamless extension of SNOMED hierarchy. Concepts without equivalency mappings are kept standard and have hierarchical relations to SNOMED concepts. No internal hierarchy for ICDO3 is created.

All ICDO3 concepts have either hierarchical or mapping relations to SNOMED.

Additionally, ICDO Condition have specific relations to ICDO Histology and ICDO Topography they originate from. Standard ICDO Conditions inherit SNOMED-specific relations to attributes from their ancestors.

ICDO3 concepts belong either to Spec Anatomic Site, Condition or to Observation domain.

Domains of concepts in ICDO3 vocabulary mimic Domains of concepts from corresponding SNOMED hierarchy.

Note: domain_id for topography concepts is currently assigned wrong and will be updated closest release

• International association of cancer registries: Primary source for histology and topography concepts
• NIH SEER: ICDO3 coding guidelines and validation list
• SNOMED CT editorial guide: used to create attribute extensions and manage hierarchy