Radiation treatment for thymoma in a dog.

A 10-year-old Toy Poodle was admitted for evaluation of a chronic cough caused by a cranial mediastinal mass. The mass was diagnosed, by needle biopsy, as a thymoma. Detection of serum acetylcholine receptor antibodies suggested that the dog had subclinical myasthenia gravis. The dog underwent orthovoltage radiation treatment, which resulted in an approximate 60% reduction in tumor mass and freedom from clinical signs for 6 months. Concurrent use of prednisolone may have been associated with disappearance of the acetylcholine receptor antibodies.