Outcome of external beam radiotherapy for treatment of noncutaneous tumors of the head in horses: 32 cases (1999-2015)

Background The long-term outcomes of external beam radiotherapy for treatment of noncutaneous tumors of the head in horses is unknown. Objective To report the long-term outcomes for treatment of noncutaneous tumors of the head of horses, and report short and long-term clinical adverse effects. Animals Thirty-two horses treated in 2 referral hospitals. Methods In this retrospective study, medical records of horses receiving radiation therapy for noncutaneous tumors between 1999 and 2015 were reviewed. Signalment, tumor type, treatment protocol, tumor control duration, and survival were recorded. Kaplan-Meier survival curves were generated for overall survival (OS), by tumor type and location, and compared using Log-rank tests, and treatment protocol adherence. Results Follow-up ranged from 2 to 145 months (median 14 months). Of 32 horses, 16 (50%) were alive at the time of reporting, with complete tumor response occurring in 12 (38%). Horses with tumors of the maxilla/nasal cavity had significantly shorter median OS compared to horses with tumors in other locations (21 months vs 145 months) (P = .06). Adverse effects resulting from the tumor or the therapy occurred in 20/32 (63%). The occurrence of major adverse effects and delays in treatment protocol were not significantly associated with median survival estimates. Conclusions and Clinical Importance External beam radiotherapy can be used to treat a variety of noncutaneous tumors of the head of horses. Adverse effects related to radiotherapy or the tumor are common.