The Pathologist's Role in Oncology: A Review of Diagnostic Approaches to Common Tumours in Dogs

Cancer is among the most significant causes of morbidity and mortality in domestic dogs, and the veterinary pathologist occupies the central and irreplaceable role in its recognition, classification, grading, and characterization. The accurate histopathological diagnosis of canine tumours forms the definitive foundation upon which all clinical oncological decision-making is built—from surgical planning and systemic staging through to chemotherapeutic protocol selection and prognostic communication. This review examines the pathologist's diagnostic role across the most clinically prevalent and pathologically instructive tumour types in dogs, including mast cell tumours (MCTs), lymphoma, mammary gland tumours, soft tissue sarcomas (STS), and osteosarcoma. For each tumour category, the review addresses the pathological basis of classification, the histological grading systems available and their clinical utility, the application of ancillary techniques including immunohistochemistry (IHC), flow cytometry, polymerase chain reaction for antigen receptor rearrangements (PARR), and next-generation sequencing (NGS), and the integration of pathological findings with clinical staging and therapeutic planning. The increasing application of digital pathology tools and artificial intelligence to canine tumour diagnosis is also examined. This review argues that the veterinary pathologist's contribution to canine oncology extends well beyond the production of a diagnostic label: it encompasses the full characterization of tumour biological behaviour, the identification of molecular targets for therapy, and the provision of prognostically meaningful data that directly guides patient management.