Incorporation of Biologic Variables Into the Staging for Canine Cutaneous and Subcutaneous Mast Cell Tumours: Proposal of the <scp>UBo pTNM</scp> System

ABSTRACT Canine cutaneous mast cell tumours (MCTs) are currently staged based on the World Health Organization (WHO) classification, which has remained unchanged since its initial formulation. Our study aimed to assess the reliability of a novel pTNM staging system, which incorporates tumour extent (T), lymph node involvement (N), presence of distant metastases (M) and the two‐tier histologic grade. We analysed medical records of dogs with one or more cutaneous/subcutaneous completely staged MCT, undergoing tumour excision with lymphadenectomy, unless distant metastases were present, in which cases, medical therapy was administered. Dogs were categorized into three stages: I (T1‐2N0M0), II (T1‐2N1M0) and III (distant metastases). Stages I and II were further divided based on histologic grade into ‘low’ and ‘high’. Substage b was defined as the presence of tumour diameter of ≥3 cm and/or ulceration. Of 226 dogs, 87 (38.5%) were in Stage I (I‐low, n = 75; I‐high, n = 12), 107 (47.3%) in Stage II (II‐low, n = 59; II‐high, n = 48), and 32 (14.2%) in Stage III. The newly proposed staging system was able to significantly stratify the population for both time to progression and tumour‐specific survival. Compared to Stage I‐low, the risk of progression increased significantly for Stage I‐high (18.3 times), Stage II‐low (8.5 times), Stage II‐high (41.5 times) and Stage III (110.3 times). The staging system was highly prognostic for both cutaneous and subcutaneous MCTs. Prospective validation studies are essential to compare this new system with the current WHO staging and further validate its accuracy and clinical utility.