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This article evaluates the responses of 14 dogs with brain masses using orthovoltage irradiation for definitive treatment. Dogs were anesthetized for computed tomography (CT) examination, formation of head immobilization and positioning devices, radiation treatment simulation, and treatments. Total doses of 39 Gy (9 dogs) or 45 Gy (5 dogs) to the tumor were administered over 25 to 41 days. Two or three portals (parallel opposed lateral with or without a dorsal field) were used. Treatment volumes included the tumor and peritumoral edema, as determined by CT scan, and a 1-cm margin. Histopathologic diagnoses were available in 9 of 14 dogs. There were 4 meningiomas, 1 lymphosarcoma, 1 pituitary adenoma, 1 metastatic anaplastic carcinoma, 1 anaplastic oligodendroglioma and 1 dog with granuloma-tous meningoencephalitis. At the end of radiation therapy, 10 dogs could be evaluated for progression of clinical signs: 3 dogs deteriorated or failed to improve, and 7 dogs improved. At the time of analysis, all dogs were dead. Mean and median survival times, measured from the beginning of radiation, were 345 and 489 days, respectively. This was compared with mean survival times of 30 to 81 days reported in the literature for dogs with brain tumors that did not receive treatment. The median survival time of 9 dogs treated with 39 Gy was 153 days, versus 519 days for 5 dogs that received 45 Gy. It appears that radiation therapy prolongs survival times for dogs with brain masses. Although megavoltage therapy would be optimal, orthovoltage radiation can be applied in total doses of 45 Gy in 3.75 Gy fractions over 28 days without adverse effects. Histopathologic evidence of multifocal demyelination and astrocytosis may be found. (Journal of Veterinary Internal Medicine 1993; 7:216–219. Copyright © 1993 by the American College of Veterinary Internal Medicine.)