## 2 Materials and Methods
### Vocabulary Mapping  
ICD-O3 is a combined classification of the topography and morphology of a tumor [18]. The topography is derived, in part, from the ICD-10 code and has a 4-digit character that covers the range from C00.0 to C80.9 which, similar to ICD-10, specifies the tumor category. The Morphology code of ICD-O3 specifies the type of cell of the neoplasm and the behavior. The ICD-O3 is implemented in the CDM in the Condition Domain and links the Condition_occurrence events with the disease episodes of the oncology module. The North Association of Central Cancer Registries (NAACCR) defines cancer registry standards for the structured acquisition of data in North America [19]. NAACR incorporates existing ontologies and classifications, such as ICDO-3, into its data standards. This ontology is mainly used in cancer registries in the USA and Canada. All data collected in the context of cancer therapy and diagnosis are assigned to specific items, which are either superordinate or assigned to special schemes, according to the respective cancer entity. Each item has a NAACCR value. Source items were mapped according to NAACR at item and value level. The National Library of Medicine (NLM), which is part of the National Institutes of Health (NIH) of the USA, provides information and research services for making biomedical data usable in the context of healthcare, and grants access to evidence-based results [20]. In 2003, the NLM developed and administered the ontology SNOMED-CT (Systematized Nomenclature of Medicine Clinical Terms) [21]. It has nine hierarchically arranged concepts, of which this study uses Clinical Finding and Procedure, covering hierarchical levels 1 and 2. By incorporating the root concept, the underlying subtypes can be identified with their associated descendants. The higher the concept class of the corresponding domain, the more descendants can be identified in SNOMED-CT. However, it is also possible to infer the root concept from the descendants. HemOnc is a medical Wiktionary. It provides information on treatment regimens, subdivided by disease subtypes, and additionally offers information on drugs, interventions, and general information on the treatment of neoplasms [22]. The HemOnc Wiki was integrated into the Standardized Vocabulary Metadata Repository v5 to provide a link between the abstraction of Treatment Regimen Episodes of the oncology module and low-level drug events of the OMOP CDM [23]. As part of the Episode implementation, source data were mapped to the new vocabulary (Figure 2, Table 1). In the OMOP CDM, the ICDO-3 classification was used to represent the cancer diagnosis. The elements that were used to specify the tumor diseases in more detail were included in the domains of Measurement and Observation. As part of the implementation of the Episode Domain, the source data was mapped to ICDO-3, SNOMED, ATC, HemOnc, Cancer Modifier, and NAACCR standardized vocabularies. Thereby, the primary approach was to map the oncological data to the SNOMED-CT terminology. If another classification system was more granular, with respect to cancer representation, it was preferred to SNOMED-CT.