External beam megavoltage radiation therapy
External beam megavoltage radiation therapy is the most effective and best described adjuvant treatment currently used for
managing incompletely resected cutaneous mast cell tumors (Figure 6). To adequately treat localized, microscopic disease, individual doses of radiation, or fractions, are repeatedly delivered
to the tumor site and margins. The therapeutic goal of radiation therapy can be categorized as either curative-intent or palliative;
these two treatment intents differ in the total number of fractions delivered and the amount of radiation energy administered
per treatment fraction.
FIGURE 6. This German shepherd was treated with surgical excision followed by curative-intent radiation therapy for a mast
cell tumor involving the metatarsal area. The alopecic area and abnormal looking skin represent early self-limiting side effects
of radiation therapy.
Curative-intent radiation involves delivering multiple sequential fractions, usually every day or every other day, for three
to six weeks. As implied, the purpose of curative-intent radiation is to provide durable and definitive local tumor control.
Patients receiving radiation therapy are sedated or anesthetized during each treatment session. Adequate patient immobilization
ensures the accurate delivery of radiation therapy to the target site, allowing for maximal tumor kill and minimal adverse
effects to normal surrounding tissues. However, radiation side effects are common in dogs receiving curative-intent treatment
and may include moist desquamation, hair loss, pain, inflammation, and localized erythema.
In several reports, curative-intent radiation therapy has been demonstrated to be effective in treating incompletely resected
cutaneous mast cell tumors in dogs.16-19 In these clinical studies, curative-intent radiation therapy for microscopic residual disease prevented local tumor recurrence
in most (about 90% to 95%) of the dogs treated.
In addition to its effectiveness in treating localized microscopic disease, curative-intent radiation therapy is useful for
treating regionally metastatic cutaneous mast cell tumors. In one report, grade III cutaneous mast cell tumors with or without
regional lymph node metastasis, were treated with curative-intent radiation. Both incompletely resected microscopic disease
and macroscopic tumor burden within the affected lymph nodes received curative-intent radiation therapy. In this study, dogs
receiving radiation therapy achieved a median survival time of 28 months. However, most tumor-related deaths were attributed
to regional disease progression, emphasizing the limitation of curative-intent radiation therapy for treating metastatic,
high-grade mast cell tumors.20
In a second study, dogs with incompletely excised cutaneous mast cell tumors with regional lymph node metastasis were treated
with a combination of oral prednisone and curative-intent radiation therapy. In that study, dogs receiving radiation to both
the primary tumor site and metastatic lymph nodes, in conjunction with oral prednisone therapy, achieved an impressive median
survival time of 1,240 days.21 These results may suggest that incompletely excised, high-grade mast cell tumors and their associated regional metastasis
are best treated with a combination of adjuvant therapies.
Although curative-intent radiation therapy is effective for treating residual microscopic cutaneous mast cell tumors, the
need for special facilities and associated high treatment costs have limited its use to pet owners with adequate financial
means and a willingness to seek treatment from veterinary specialists. Because of these limitations, other adjuvant treatment
options have been evaluated for treating residual cutaneous mast cell tumors, including the use of systemic chemotherapy.
The efficacy of systemic chemotherapy for treating measurable canine mast cell tumors has been recently reported.22-26 Intuitively, if chemotherapeutic agents possess efficacy against measurable cutaneous mast cell tumors, it would be expected
that these same antineoplastic drugs would also be effective in managing residual microscopic disease. Unfortunately, prospective
clinical trials evaluating the efficacy of systemic chemotherapy or oral prednisone for treating residual microscopic disease
are few. In one study, seven dogs with residual microscopic disease were treated with prednisone and vinblastine in lieu of
curative-intent radiation therapy.22 The median survival time of these seven dogs was more than 1,013 days, thereby suggesting the therapeutic efficacy of prednisone
and vinblastine used in an adjuvant setting for the treatment of residual mast cell tumor disease. To better support the adjuvant
use of systemic chemotherapy, a recent report evaluated the therapeutic effectiveness of prednisone and vinblastine in 27
dogs with inadequately excised cutaneous mast cell tumors (24 grade II, three grade III). In that study, 20 dogs available
for follow-up were evaluated for local and distant tumor growth after a median of 537 days. Two dogs (10%) experienced local
tumor regrowth, but four additional dogs developed new mast cell tumors involving distant cutaneous sites.27