Efficacy of cobalt 60 radiotherapy in dogs with pituitary-dependent hyperadrenocorticism

Objective-To determine efficacy of cobalt 60 radiotherapy in dogs with pituitary-dependent hyperadrenocorticism (PDH) that have detectable tumors but no neurologic abnormalities. Design-Case series. Animals-6 dogs with PDH that had a detectable pituitary mass an magnetic resonance images. Procedure-Radiation was delivered in 11 fractions during a 3.5-week period for a total dose of 44 Gy. Clinical signs were evaluated, a urinalysis and ACTH stimulation test were performed, and urine cortisol-to-creatinine ratio and plasma endogenous ACTH concentration were measured before, immediately after, and 1, 3, 6, 9, and 12 months after radiotherapy. Magnetic resonance imaging was repeated 1 year after radiotherapy. Results-Clinical signs of hyperadrenocorticism resolved in 3 dogs but recurred in 2 of the 3, Clinical condition of 2 dogs improved but did not return to normal. One dog did not improve. Results of ACTH stimulation tests and urine cortisol-to-creatinine ratios correlated with clinical signs. Plasma endogenous ACTH concentration transiently decreased in all 6 dogs, One year after radiotherapy size of tumors was decreased by 25% in 2 dogs; in the other 4 dogs, tumors could no longer be detected. None of the dogs developed neurologic abnormalities. Adverse effects of radiotherapy were mild. Clinical Implications-Radiotherapy did not result in adequate control of clinical signs of hyperadrenocorticism in 5 of 6 dogs, but size of pituitary tumors was dramatically reduced. Thus, it may be reasonable to recommend radiotherapy in dogs with PDH that have pituitary tumors for which greatest vertical height is 8 mm or more.