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.

1. ICDO Topography Concepts that represent various body sites.

ICDO Topography concepts generally correspond to codes of malignant neoplastic condition concepts in ICD10 vocabulary and use the same format 4 character alphanumeric codes.

CONCEPT_CODE	VOCABULARY_ID	CONCEPT_NAME	CONCEPT_CLASS_ID
C15.2	ICDO3	Abdominal esophagus	ICDO Topography
C15.2	ICD10	Malignant neoplasm: Abdominal part of oesophagus	ICD10 code
C15	ICDO3	Esophagus	ICDO Topography

Benign neoplasms, in-situ neoplasms or neoplasms of uncertain behavior will still use the same ICD10 codes for malignant neoplasm to represent topography. Conversion tables from ICD10 conditions to ICDO3 topography codes are provided by SEER.

All topography concepts are valid and are either standard or mapped to Standard SNOMED concepts from 'Spec Anatomic Site' Domain.

2. ICDO Histology 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 encode 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.

Outdated or unconfirmed histology concepts are deprecated in OMOP standard vocabularies but have Concept replaced by links to active counterparts whenever possible. All valid Histology concepts are either mapped to Standard concepts in SNOMED or are Standard themselves.

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

Codes of ICDO conditions are formed by concatenating Histology and Topography codes over hyphens. Condition names are concatenated from names of their attributes without any modifications from names of topography and histology. Source data is not expected to have neither these combined codes nor terms: usually codes for Histology and Topography are provided in separate fields.

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.

CONCEPT_CODE	CONCEPT_NAME	NOTE
8933/3-NULL	Neoplasm defined only by histology: Adenosarcoma	Condition misses topography definition
NULL-C50.9	Neoplasm defined only by topography: Breast, NOS	Condition misses histology definition
8560/3-C34.9	Adenosquamous carcinoma of lung, NOS	Fully defined condition

Note: If condition definition misses histology attribute, it is often beneficial to treat topography code as pure ICD10 code

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. All SNOMED conditions that have exact match on attribute set are taken as mapping targets and all concepts that have more generic attribute set are made ancestors. Existing mappings, list of combinations and logic for building the condition hierarchy can be found on projects github.

Conditions built from outdated or unconfirmed histologies may still be standard with hierarchy participation if no other mapping is available.

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. Condition concepts without equivalence mappings are kept standard and have hierarchical relations to SNOMED concepts.

Hierarchical levels of both topography and histology attributes are present and have hierarchical relationships to subsuming attributes. Hierarchical attributes are valid, non-standard and do not participate in combination forming.

All ICDO3 concepts excluding hierarchical levels of attributes have either hierarchical or mapping relations to SNOMED concepts.

Additionally, ICDO Condition have specific relations to ICDO Histology and ICDO Topography they originate from. Standard ICDO Conditions inherit and mimic 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.

ICDO concept_class_id	SNOMED concept_class_id	Shared domain_id
ICDO Topography	Body Structure	Spec Anatomic Site
ICDO Histology	Morph Abnormality	Observation
ICDO Condition	Clinical Finding	Condition

• 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
• Conversion tables from ICD10 conditions to ICD-O topography: Recommended for use for ETL