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Radiation therapy can be used as part of multimodal therapy either as an adjunctive therapy after incomplete surgical MCT excision or as a primary treatment modality of the primary tumor and/or regional lymph node(s). Radiation is most effective and most commonly used as an adjunctive therapy after surgical removal or tumor size reduction to a microscopic level (LaDue et al. 1998). When possible, surgery should be performed prior to radiation therapy to decrease the tumor volume, as dogs treated with adjuvant radiation therapy with smaller tumor volumes have longer disease‐free intervals than those with larger tumor volumes (LaDue et al. 1998; Hahn et al. 2004). The risk of systemic effects because of MCT degranulation is present in tumors treated with radiation of macroscopic disease, so pretreatment with prednisolone is recommended (Dobson et al. 2004).

Radiation therapy is very effective at eliminating residual microscopic disease after incomplete excision of grade I and II MCTs. Local tumor control rates of 86–94% at two and three years are reported after adjunctive radiation therapy for incompletely excised grade II MCTs (Frimberger et al. 1997; al‐Sarraf et al. 1996; Poirier et al. 2006). These rates are similar to the local tumor control rate of 89% achieved with complete surgical excision of grade II MCTs (Weisse et al. 2002). Radiation therapy for incompletely excised grade III, stage 0 MCTs, is encouraging compared to untreated incompletely excised grade III MCTs with a one‐year local control rate of 65% and one‐year survival rate of 71% (Hahn et al. 2004).

If the regional lymph node is positive for MCT disease (stage II disease), radiation therapy of the affected lymph node has been advocated by some investigators to improve survival time. A study by Poirier et al. found no difference in overall survival rate, whether the regional lymph node was prophylactically irradiated or not (Poirier et al. 2006).

Palliative radiation using 4 × 8 Gy fractions at weekly intervals has been reported as a treatment option for unresectable MCTs alone or in combination with chemotherapy (Dobson et al. 2004).