Mast cell tumors (MCTs) account for 7% to 21% of all cutaneous and subcutaneous tumors in dogs. The degree of malignancy varies, and treatment approaches are controversial. The standard of care for localized grade II MCTs is wide surgical excision without adjuvant therapy. However, the rate of local recurrence after complete excision has been reported to be between 3% and 11%. Because the recurrence rate for incompletely excised MCTs is higher (18% to 35%), additional treatment-which may include further surgery, radiation therapy and/or chemotherapy-is usually indicated. Histologic grade continues to be the most useful predictor of biological behavior of MCTs; other factors evaluated have included clinical stage; growth rate; systemic signs; location; and proliferation indices, such as argyrophilic nucleolar organizer regions (AgNORs), proliferating cell nuclear antigen (PCNA), and Ki-67-positive nuclei. However, grade II MCTs continue to be difficult to prognosticate compared with grade I and III tumors. This retrospective study examines local recurrence rate; clinical outcome; and prognostic value of the number of AgNORs, presence of PCNA, and number of Ki-67-positive nuclei after incomplete surgical excision of canine cutaneous grade II MCTs. Only patients with incomplete excision and that received no ancillary therapy (except prednisone) were studied. Twenty-eight dogs with 30 MCTs were evaluated. Twenty-three percent of the tumors recurred locally, and 39% of dogs developed MCTs at other locations. Median overall survival time was 1426 days. Both Ki-67 and PCNA scores were prognostic for local recurrence, and dogs with local recurrence had decreased overall survival times.

COMMENTARY: This retrospective study questions the benefit of adjuvant treatments for incompletely excised grade II MCTs based on the fact that even with adjuvant therapy, local recurrence has been reported to be between 5% and 20% depending on the study and type of therapy. The results of the authors' proliferation studies were similar to those of others. The authors note the limitations of this study, such as small sample size and limited availability of information for some patients. A recurrence rate of 23% could be considered high, especially because local recurrence was a negative prognostic indicator for overall survival of these patients. Treatment of these types of tumors will remain controversial until further studies that better predict which tumors should be treated with adjuvant therapy are completed. Until then, the standard of care is recommended, but ultimately, it is a decision that needs to be made between the client and the veterinarian with the patient's best interest in mind.

Recurrence rate, clinical outcome, and cellular proliferation indices as prognostic indicators after incomplete surgical excision of cutaneous grade II mast cell tumors: 28 dogs (1994-2002). Seguin B, Faulkner Besancon M, McCallan JL, et al. J Vet Intern Med 20:933-940, 2006.