Megavoltage irradiation of pituitary macrotumors in dogs with neurologic signs.

OBJECTIVE

To assess the efficacy and determine prognostic factors of megavoltage irradiation for pituitary macrotumors in dogs with neurologic signs.

DESIGN

Prospective clinical trial.

ANIMALS

24 dogs with pituitary macrotumor syndrome; 19 ACTH-secreting and 5 clinically endocrine-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 progression of neurologic signs.

RESULTS

A significant correlation was found between relative tumor size (i.e., size of tumor relative to calvarium size) and severity of neurologic signs and between relative tumor size and remission of neurologic signs after irradiation. In dogs with pituitary-dependent hyperadrenocorticism, a significant correlation was found between relative tumor size and plasma endogenous ACTH concentrations. Prognostic factors that independently affected duration of remission of neurologic signs were relative tumor size and endocrine activity. The prognostic factor that independently affected overall survival time was severity of neurologic signs. Prognostic factors of duration of eucortisolism were not found. Use of a large field of irradiation was associated with substantial damage to brain tissue.

CLINICAL IMPLICATIONS

Because radiation therapy was effective for treatment of tumors of small relative size in dogs, early treatment of pituitary tumors should improve prognosis. Further improvements may be obtained, using protocols in which higher total radiation doses and smaller radiation dose fractions are given. Irradiation was effective for long-term control of functional pituitary macrotumors and resulted in acceptably low complication rates when small fields of radiation were used.