# A Comparative Study of Melanocytic Tumours: Linking Portuguese Dogs and Cats to Human Cases

## TL;DR

This study compares melanocytic tumors in Portuguese dogs, cats, and humans, highlighting similarities and differences in incidence and breed susceptibility.

## Contribution

The study provides the first epidemiological comparison of melanocytic tumors across three species in Portugal.

## Key findings

- Melanoma was the most frequently diagnosed melanocytic tumor in all three species.
- Shar-Peis had the highest relative risk of melanocytic tumors compared to mixed-breed dogs.
- There was significant geographical overlap in tumor cases between humans and dogs in urban regions.

## Abstract

Melanocytic tumours (MT) occur in both humans and companion animals, presenting an opportunity for comparative oncology research. Thus, this study provides a comprehensive epidemiological analysis comparing MT in Portuguese dogs, cats and humans. Data were obtained from the Portuguese National Cancer Registry (RON) (2011–2021) and Vet‐OncoNet (2020–2023), utilising standardised oncological classification systems (ICD‐O‐3.2 and Vet‐ICD‐O‐canine‐1). The results indicate that Melanoma was the most frequently diagnosed MT across all three species, while melanocytomas were common in dogs but rare in cats and humans. A higher incidence rate (IR) for MT was observed in dogs (IR = 16.1) compared to humans (IR = 8.1) and cats (IR = 6.3), and neutered dogs (10.8 years) were diagnosed at significantly older ages than intact ones (9.9 years). Shar‐Peis (RR = 14.2, p < 0.001) had the highest RR compared to mixed‐breed dogs, followed closely by Rhodesian Ridgebacks (RR = 12.2, p < 0.001) and Golden Retrievers (RR = 6.4, p < 0.001). Spatial analysis revealed significant clustering of MT cases in humans and dogs, with a strong geographical overlap (BLISA = 0.345, p < 0.001) in urban regions. This study provides the first epidemiological comparison of MT in these three species in Portugal, underscoring the sentinel role of companion animals in human oncology and the relevance of comparative oncology in translational cancer research.