| Summary: | There are increasing efforts directed at providing formal frameworks to consolidate the widening net of terms and relations used in medical practice. While there are many reasons for this, the need for standardisation of protocol and terminology is critical, not only for the provision of uniform levels of health care, but also to facilitate medical science research. In the domain of breast cancer pathology, a summary of current practice by the World Health Organisation states that the variability of the evidence archive (inconsistencies in describing microscopic appearances of phenomena, different diagnostic thresholds for working pathologists) is chief among the barriers to the medical understanding of the symptoms and development of early cancers. Such variability is acknowledged across specialist fields of medicine, motivating standardisation of terminologies for reporting medical practice. The desideratum of making these standards machine-readable has led to their formalisation as ontologies. Ontologies are computational artefacts designed to provide representations of a domain of interest. Thus, the representation must be a formal description so that it can be encoded, and reused, allowing navigation of the key concepts recorded and retrieval of information indexed against it. This brings the required standardisation by offering a set of labelling options to record observations and events encountered by medical professionals. Given the twin goals of ontologies -- representation and standardisation -- this paper will consider the key question of their design in the context of the use by experts, of information handling applications built around them. We build on our experience in developing ontologies for decision support software in the area of breast cancer diagnosis and treatment. We will also examine, from this perspective, the suggestion offered in the literature that a set of metaphysically motivated questions should form the basis of ontology building as guarantors of fidelity to reality. We find that ontologies intended to support medical practice can only be understood within the context of their intended use. The declarative framework within which they are encoded generates the hope that their meaning transcends the specific application context. We show, however, that these declarative statements are to be understood as end products of chains of procedural engagements between humans, materials and communitarian norms. It is only when this scaffolding that brings this representation into existence becomes routine and consensual (within the community that exchanges information indexed against it) that the concepts stand in for physiological states with independent dynamics. However, as the state of biomedical knowledge is always in a state of flux, and different institutions and practitioners may be out of sync with respect to such modifications, the concepts embedded in the ontologies are constantly subject to reinterpretation within the context of specific institutional practices. Given the fragmentation of the patient's body when viewed through various specialised lenses, ontologies can provide placeholders for co-ordinating disparate viewpoints to provide suitable medical interventions. The extent to which such interventions reflect any underlying reality, as manifest in measures of their efficacy, is closely wrapped up in the regulatory apparatus of protocol-guided consensus making. The value of ontologies lies in their reflection of, and support for, the sense-making activities that constitute expertise, not in their transparent access to a metaphysical reality.
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