Design of the TNM-O

The relation between the artefacts of the TNM classification and the actual tumour diseases is denotational: the T code denotes the extent (size, infiltration) of the primary tumour, the N code the extent of regional lymph node metastases, and the M code the existence of distant metastases. For TNM-O, we adopted an approach which is compliant with the Information Artefact Ontology from the OBO Foundry and recently published work on the aboutness relation [44, 45]. In TNM-O, coding artefacts of the TNM classification i.e. the classes of the classification are represented by subclasses of btl2:InformationObject as RepresentationalArtefact. Information reported on individual patients, e.g. as TNM-codes in patient records are thus individuals of these classes. Individuals from subclasses of InformationObject are related by btl2:represents to individuals of classes about the current disease state (AnatomicalStructure). The inverse relation is btl2:isRepresentedBy connects material or processual entities with the respective TNM-artefact.

As the TNM classification is compositional, the individual classes of the three descriptors can be independently combined to a joint code. Classes are only dependent on the location of the primary tumour and additional modifiers c or p: e.g. cN1 for colon cancer has a different meaning than cN1 for breast cancer, and cT1 has a different meaning than pT1 for all locations where these codes are available). This characteristic is conserved in TNM-O. The class RepresentationalUnit is a superclass of organ specific classes separated in a clinical and a pathological branch.

For representing anatomical structure, TNM-O uses content from the Foundational Model of Anatomy, restricted to cancer-related anatomy as referred to by the TNM classification. All primary tumours individuals and metastases are then related to individuals anatomical entities by the relation btl2:locatedIn, thus providing them with an exact topography and extent. The extent of primary tumours cannot only be described by their localisation (i.e. occupying space or infiltrating through layers of an organ) but can be further characterised by qualities, e.g. tumour size or infiltration patterns. These qualities are dependent on the localisation of the primary tumour and can substantially differ between them.

What makes a lymph node a regional lymph node depends on its proximity to a primary organ. An axillary lymph node is a regional lymph node of the breast gland but not of the colon. For all relevant organs, these regional lymph node groups are to be defined. Moreover, the formalisation of infiltrated regional lymph nodes depends on the aggregate of a localised primary tumour together with some metastasis in a regional lymph node of that organ in which the primary tumour is located. Thus, an infiltrated axillary lymph node is a regional lymph node metastasis for a breast tumour, but certainly not for a colon cancer. Distant metastases are, by definition, those located in a tumour aggregate that is not a regional lymph node of the primary tumour.