Canine mast cell tumours: assessment, staging and surgical treatment

Mastocitoma canino are malignant tumours of the skin and subcutaneous tissue. They are one of the most common skin neoplasms reported in dogs, with an incidence of 7–21%. It tends to appears around the age of 8 years and is more frequent in breeds such as the Boxer, Bulldog, Bullmastiff, Pitbull, Labrador, Golden Retriever, Dachshund and American Staffordshire Terrier.

At the time of diagnosis, many dogs already have regional or distant lymph node metastases, especially in the liver or spleen. Although advances in diagnostic techniques have shed more light on this type of tumour, its variable biological behaviour mean its evolution is hard to predict, thus complicating the choice of treatment.

Histological grading of canine mast cell tumours to help predict their evolution

The histological grading of malignancy is a key factor when classifying cutaneous mast cell tumours. The Patnaik classification ¹ has traditionally been used to predict the evolution of mast cell tumours in dogs. In fact, it can be used to reliably predict the course of grade I and III mast cell tumours, but not grade II tumours, many of which present an aggressive biological behaviour.

A group of pathologists from the University of Bologna ² has proposed a new classification (Kiupel) that divides mast cell tumours into two groups of either low or high grade. The researchers used both systems to assess the histological grading of 137 surgically removed cutaneous mast cell tumours and patient survival.

According to the Kiupel system, all grade I canine mast cell tumours were considered low grade, and all grade III tumours were high grade. Of the grade II tumours, 85.6% were classified as low grade and 14.4% were high grade, with 1-year survival probabilities of 94% and 46%, respectively.

This new classification has a high prognostic value as it can be used to correctly predict the negative results of some grade II mast cell tumours. It indicates that a high-grade mast cell tumour must meet at least one of the following criteria in 10 high-power fields:

• 7 mitotic figures
• 3 or more multinucleated cells
• 3 or more abnormal nuclei (atypical, segmented, irregular)
• Cariomegaly, at least 10% of the neoplastic cells have nuclei that are double their normal size

The mean survival time is 2 years for low-grade mast cell tumours. The 1-year survival probability was 95% for dogs with low-grade tumours and 24% in the case of high-grade tumours. The mean survival time for high-grade tumours was 150 days.

Combining both classifications can help determine a more accurate prognosis for grade II mast cell tumours, so another study conducted at the University of Illinois³ recommends vets refer to both.

Postoperative evolution of mast cell tumours

An early diagnosis and aggressive surgery are essential for the successful treatment of canine mast cell tumours. Surgery is usually successful if the mast cell tumour is small, clearly differentiated and localised, but it is vital that the veterinary surgeon leaves clean tissue margins.

A 2‑cm lateral surgical margin and a depth of 1 fascial plane are recommended, according to a study published in the Journal of the American Veterinary Medical Association⁴. The regional lymph node must be removed if metastasis is suspected. Conservative surgery, however, presents a 50% chance of recurrence. Obviously, the risk of recurrence is greater in high-grade tumours, even with clean margins.

Finally, a study conducted at Colorado State University⁵ analysed the clinical progress of dogs with grade II mast cell tumours treated exclusively through surgery. After analysing 55 dogs, the authors found that 84% did not suffer a mast cell tumour recurrence and only 5% developed metastases, so they concluded that most dogs do not need systemic therapy after complete surgical excision of grade II tumours.

1. Patnaik, A.K. et al. (1984) Canine cutaneous mast cell tumor: morphologic grading and survival time in 83 dogs. Vet Pathol; 21(5): 469-474.

2. Sabattini, S. et al. (2015) Histologic grading of canine mast cell tumor: is 2 better than 3? Vet Pathol; 52(1): 70-73.

3. Reagan, J.K. et al. (2018) Evaluation of information presented within mast cell tumour histopathology reports in the United States: 2012-2015. Vet Med Sci; doi: 10.1002/vms3.107.

4. Fulcher, R.P. et al. (2006) Evaluation of a two-centimeter lateral surgical margin for excision of grade I and grade II cutaneous mast cell tumors in dogs. J Am Vet Med Assoc; 228(2): 210-215.

5. Séguin, B. et al. (2001) Clinical outcome of dogs with grade-II mast cell tumors treated with surgery alone: 55 cases (1996-1999). J Am Vet Med Assoc; 218(7): 1120-1123.