A new study from CVS, published in the Journal of Veterinary Internal Medicine, has provided important new insights into how veterinary professionals can better predict outcomes for dogs diagnosed with mast cell tumours (MCTs) - one of the most common forms of skin cancer in dogs.

The study, titled Clinical Value of Various Histological Factors in Cutaneous and Subcutaneous Mast Cell Tumours in 197 Dogs, analysed 199 tumours from 197 dogs treated surgically in primary care practices. The aim was to assess the value of a wide range of histological and molecular markers that are currently available in commercially available prognostic panels in predicting survival and tumour recurrence, especially in cases where traditional grading systems offer limited clarity.

MCTs make up around 16 to 20 percent of all skin and subcutaneous tumours in dogs. While grading systems like Patnaik and Kiupel are commonly used to assess tumour prognosis, they don’t always give clear answers - particularly in view of the majority of MCTs being Patnaik intermediate and Kiupel “low” grade; categories which are known for a spectrum of tumour behaviours. Performing additional prognostic markers may help, but these still may not give a clear picture or markers may even contradict each other – with some indicating a poor prognostic outcome and others a fair outcome. Many previous studies exploring prognostic markers have only examined these markers separately, therefore this is the first study of this size to have compared the value of these various markers, and has helped us to understand which marker may have the best predictive ability when conflicting results are presented.

The study examined a broad panel of prognostic markers. These included mitotic count (a measure of how quickly cells are dividing), Ki67 (a proliferation marker), AgNOR (a marker of nucleolar activity), KiAg (a combination of Ki67 and AgNOR), KIT staining pattern, and genetic mutations in the c-kit gene (specifically exons 8 and 11). The goal was to identify which of these markers could independently predict mast cell tumour-specific survival (MSS) and the likelihood of tumour recurrence.

In agreement with many previous studies, this study showed that a higher mitotic count was a strong independent histological predictor of poor survival across all tumour types. Most interestingly however, Ki67 - often used to assess tumour activity - was not independently significant in this study, which contrasts with previous research.

Importantly, among cutaneous tumours graded as Patnaik grade I or II and Kiupel low-grade, a c-kit mutation in exon 11 was the only marker that independently predicted survival. For tumour recurrence, both AgNOR counts above 2.25 per cell and the c-kit mutation in exon 11 were independently significant - even in tumours that appeared low-risk based on traditional grading.

These findings suggest that c-kit mutation in exon 11 and AgNOR may be more clinically useful than previously thought, and Ki67 less clinically useful than previously thought. . The study also found that cutaneous and subcutaneous MCTs behaved similarly in terms of survival and recurrence, supporting the idea that they can be managed using similar treatment protocols.

The research also looked at histological tumour-free margins (HTFM) and found no significant difference in recurrence between incomplete excision and HTFM less than two millimetres. Only margins of two millimetres or more were clearly linked to lower recurrence risk. This highlights the need for careful interpretation of margin status, as close margins do not always mean the tumour will return.

Owen Davies, Specialist in Veterinary Oncology at CVS Bristol Vet Specialists and lead author of the study, said:

The Clinical Value of Various Histological Factors in Cutaneous and Subcutaneous Mast Cell Tumours in 197 Dogsresearch was conducted by Owen Davies and Katherine Boyd at CVS’ Bristol Vet Specialists, Melanie Dobromylskyj at CVS’ Finn Laboratories and Imogen Schofield, Director of Clinical Research at CVS, along with Dan O’Neill from the Royal Veterinary College (RVC) and Celia Figueroa from The Ralph Veterinary Referral Centre. It was funded by a £5,000 CVS Research Grant.

The study cohort consisted of dogs in general practice who had undergone additional prognostic testing whose clinical records were examined retrospectively using the VetCompass programme at the RVC. This likely resulted in a higher proportion of low to intermediate-grade tumours and fewer tumour-related deaths than might be seen in referral centres. However, the distribution of tumour grades was consistent with previous studies, suggesting the findings are broadly applicable.

CVS Group operates across small animal, farm animal, equine and laboratories, with over 500 veterinary practices and referral centres in the UK and Australia. CVS has invested £122 million in its sites, facilities, equipment and technology since the start of 2023, in addition to providing industry leading training and support, to give the best possible care to animals.