TREATMENTS FOR MACROSCOPIC AND METASTATIC DISEASE
Contrary to the success in treating localized disease with surgery alone or with adjuvant therapies, the effective management
of surgically nonresectable macroscopic disease or advanced distant metastatic mast cell disease remains problematic. In these
patients with extensive disease, the intent of therapy is no longer curative. Rather, therapy should be focused on reducing
tumor burden, improving quality of life, and prolonging survival times. Therapies with these defined goals are categorized
as palliative. Several palliative treatment options exist for managing noncurable mast cell tumors in dogs, including systemic chemotherapy,
palliative radiation, and intralesional triamcinolone.
Although potentially useful as an adjuvant to surgery, systemic chemotherapy has been traditionally indicated to treat metastatic
and nonresectable mast cell tumors. Several studies have identified chemotherapeutic regimens possessing marginal to moderate
therapeutic efficacy for treating advanced mast cell tumors in dogs (Table 3). The response rates, duration of remission, disease-free intervals, and survival times of dogs with metastatic or nonresectable
mast cell tumors can be quite variable, with a subset of patients maintaining a high quality of life for an extended period.
For this reason, treatment options for patients with advanced disease should not be limited solely to euthanasia.
TABLE 3. Systemic Chemotherapy for Macroscopic Mast Cell Tumor Burden
Most chemotherapeutic regimens are easy to administer, but specialized equipment is recommended (a biologic safety cabinet)
to minimize unnecessary and hazardous exposure to chemotherapeutic agents. In addition, familiarity with the handling, safety,
side effects, and administration of each anticancer agent should be considered a necessity before instituting chemotherapy.
Radiation of macroscopic tumors: Curative and palliative fractionation schemes
When cytoreductive surgeries are not an option for cutaneous mast cell tumors, using radiation therapy alone or in conjunction
with other adjuvant therapies may be considered (Figure 7). In one study, 21 dogs with macroscopic tumor burden were treated with curative-intent radiation therapy.19 As would be expected, the initial tumor volume affected the duration of response to radiation therapy. Dogs with tumor volumes
9 cm3 or less had a significantly longer disease-free interval (about 20 months) than dogs with tumor volumes 10 cm3 or greater (about four months) had. While dogs with larger tumors had a shorter duration of response, the results from this
study support the role of curative-intent radiation therapy for managing dogs with macroscopic tumor burden.
FIGURE 7. This male Labrador retriever is suffering from aggressive multicentric cutaneous mast cell disease. Therapeutic
options, including palliative radiation therapy and systemic chemotherapy, provided a partial clinical response and quality-of-life
improvement. FIGURE 8. Intralesional therapy with triamcinolone may be useful in reducing the size of mast cell tumors before
surgical resection or for shrinking macroscopic tumors for palliative purposes.
Although curative-intent radiation may have a role in treating macroscopic disease, a large time and financial commitment
is required of pet owners. In some situations, palliative radiation therapy may be considered a more appropriate treatment
option. In comparison to curative-intent treatment regimens, palliative radiation involves administering larger doses of radiation
at less frequent intervals. A typical palliative protocol would be administering one large dose of radiation every week for
four consecutive weeks.