In addition to biologic behavior, the appropriate treatment for canine mast cell tumors will also be dictated by the extent
of disease, referred to as the clinical stage (Table 2). To determine the clinical stage in dogs with mast cell tumors, a complete blood count, a serum chemistry profile, urinalysis,
a thoracic radiographic examination, and an abdominal ultrasonographic examination should be performed before therapy is begun.
Additionally, because mast cell tumors may spread to regional or distant sites, bone marrow aspirates and the cytologic evaluation
of regional lymph node, liver, and spleen samples may also be recommended as part of the routine staging procedure. For dogs
with locally confined tumors (stage 0 to 1), the treatment of grade I or II mast cell tumors should include surgery alone
or surgery in combination with curative-intent radiation therapy. For stage 0-1 grade III mast cell tumors, local treatment
options should also be implemented, but systemic chemotherapy should be recommended given the high metastatic rate associated
with grade III mast cell tumors. In dogs with stage 2-4 tumors, combining local therapies with systemic chemotherapy should
be offered in attempts to provide improved quality of life and prolonged survival times.
TABLE 2. Clinical Staging of Mast Cell Tumors
DEFINITIVE TREATMENTS FOR LOCALIZED DISEASE
Definitive surgery can be performed in most practice settings, requiring neither additional instrumentation nor specialized
equipment. Definitive surgery is most appropriate in dogs with localized mast cell tumors of low or intermediate histologic
grade (grade I or II, respectively). Surgery is also best suited for tumors involving anatomical sites amenable to wide resection.
Two recent reports describe the effectiveness of surgery alone for treating grade II, localized cutaneous mast cell tumors.12,13 The studies found that surgical resection was an effective treatment option, with recurrence in only 5% to 11% of dogs after
complete excision. These studies emphasize that complete excision is often a curative treatment option, when anatomically
feasible, for localized cutaneous mast cell tumors in dogs.
Because definitive surgery of localized mast cell tumors may be curative, defining the extent of surgical margins required
for complete excision is important. A recent study assessed the surgical margins necessary for completely excising grades
I and II cutaneous mast cell tumors greater than 1 cm in diameter. The results suggest that complete excision of cutaneous
mast cell tumors should be achievable with lateral surgical margins of 2 cm and deep surgical margins including one fascial
Although obtaining complete margins should be the goal of any definitive surgical procedure, one recent study did not find
any difference between local tumor recurrences in dogs with histologically tumor-free vs. nontumor-free margins. This unexpected
finding may be due to the small sample population analyzed, so strong conclusions should not be drawn from this study.15 In addition, when looking at surgical margins for mast cells, normal mast cells often cannot be differentiated from neoplastic
ones, complicating the interpretation of clean vs. dirty surgical margins.
ADJUVANT TREATMENTS FOR LOCALIZED OR REGIONAL LYMPH NODE METASTATIC DISEASE
Not all dogs with localized mast cell tumors are candidates for curative surgery. In some situations, surgery may only be
cytoreductive, thereby leaving behind either residual microscopic or even macroscopic disease (Figure 5). For these patients, instituting adjuvant therapies should be recommended to decrease the likelihood of local tumor regrowth.
Best suited for the management of small tumor burdens, adjuvant therapies should be instituted shortly after cytoreductive
surgeries before measurable tumor regrowth. Although the most effective adjuvant therapy for treating canine cutaneous mast
cell tumors is external beam megavoltage radiation therapy, other treatment options such as systemic chemotherapy, intraregional
deionized water, and interstitial brachytherapy have been investigated.
FIGURE 5. A large and invasive mast cell tumor involving the left tarsus in a female Labrador retriever. The size and location
of the tumor precluded complete surgical excision. The dog was treated with cytoreductive surgery, and the residual microscopic
disease was cured with curative-intent megavoltage radiation therapy.