A Multi-Institutional Retrospective Study of 21 Dogs Having Undergone Hypofractionated Radiotherapy for Adrenal...

Radiation therapy (RT) has emerged as a promising non-surgical approach for treating canine adrenal tumours. This multi-institutional, retrospective study describes clinical outcomes for 21 dogs having been prescribed a course of hypofractionated image-guided intensity-modulated RT (IMRT) entailing delivery of 25-35 Gy total in 5 fractions given over 5-15 days for an adrenal tumour. Diagnoses were based on imaging (abdominal ultrasound or computed tomography) and biochemical testing. All dogs had unilateral or bilateral irregular adrenal masses with evidence of vessel compression or invasion. Adrenal masses were incidentally identified in 11 dogs. The clinical diagnoses included pheochromocytoma (n = 13, 61.9%), adrenocortical adenocarcinoma (n = 2, 9.5%) and unspecified (n = 6, 28.6%). Among the 16 dogs with available follow-up imaging, the rates of partial response and stable disease were 37.5% (6/16) and 62.5% (10/16), respectively. Mild gastrointestinal side effects related to RT were reported in four dogs (19%). Early death that could have been attributable to tumour or complications of treatment occurred in two dogs (9.5%) at 21 and 52 days post-RT; one presenting with acute vomiting, and the other presenting with vomiting, tremors, and shock before death. Of the 15 deceased dogs, 4 (26.7%) died due to tumour-related causes and 11 died due to unknown (n = 2) or unrelated (n = 9) causes. The median overall survival time was 377 days, with a median follow-up time of 458 days for censored patients (n = 6). The one- and two-year survival rates were 59.4% and 34.7%, respectively. These data build upon prior published reports, demonstrating that RT can be associated with prolonged survival in dogs with adrenal tumours. Hypofractionated IMRT appears to offer a potential survival benefit even in dogs with major vessel invasion or comorbidities. Future research should focus on identifying risk factors for early death and determining which patient populations are most likely to benefit from RT.