MEGAVOLTAGE IRRADIATION OF PITUITARY MACROTUMORS IN DOGS WITH NEUROLOGIC SIGNS

Objective-To assess the efficacy and determine prognostic factors of m egavoltage irradiation for pituitary macrotumors in dogs with neurolog ic signs. Design-Prospective clinical trial. Animals-24 dogs with pitu itary macrotumor syndrome; 19 ACTH-secreting and 5 clinically endocrin e-inactive tumors. Procedure-Dogs were treated with 48 Gy of radiation during 4 weeks on an alternate-day schedule of 4 Gy/fraction. Three ( 12.5%) dogs did not complete the planned treatment because of progress ion of neurologic signs, Results-A significant correlation was found b etween relative tumor size (ie, size of tumor relative to calvarium si ze) and severity of neurologic signs and between relative tumor size a nd remission of neurologic signs after irradiation. In dogs with pitui tary-dependent hyperadrenocorticism, a significant correlation was fou nd between relative tumor size and plasma endogenous ACTH concentratio ns. Prognostic factors that independently affected duration of remissi on of neurologic signs were relative tumor size and endocrine activity . The prognostic factor that independently affected overall survival t ime was severity of neurologic signs. Prognostic factors of duration o f eucortisolism were not found. Use of a large field of irradiation wa s associated with substantial damage to brain tissue. Clinical Implica tions-Because radiation therapy was effective for treatment of tumors of small relative size in dogs, early treatment of pituitary tumors sh ould improve prognosis. Further improvements may be obtained, using pr otocols in which higher total radiation doses and smaller radiation do se fractions are given. Irradiation was effective for long-term contro l of functional pituitary macrotumors and resulted in acceptably low c omplication rates when small fields of radiation were used.