In radiation therapy (RT) treatment planning for canine head and neck cancer, the tonsils may be included as part of the treated volume. Delineation of tonsils on computed tomography (CT) scans is difficult. Error or uncertainty in the volume and location of contoured structures may result in treatment failure. The purpose of this prospective, observer agreement study was to assess the interobserver agreement of tonsillar contouring by two groups of trained observers. Thirty dogs undergoing pre- and post-contrast CT studies of the head were included. After the pre- and postcontrast CT scans, the tonsils were identified via direct visualization, barium paste was applied bilaterally to the visible tonsils, and a third CT scan was acquired. Data from each of the three CT scans were registered in an RT treatment planning system. Two groups of observers (one veterinary radiologist and one veterinary radiation oncologist in each group) contoured bilateral tonsils by consensus, obtaining three sets of contours. Tonsil volume and location data were obtained from both groups. The contour volumes and locations were compared between groups using mixed (fixed and random effect) linear models. There was no significant difference between each group's contours in terms of three-dimensional coordinates. However there was a significant difference between each group's contours in terms of the tonsillar volume (P < 0.0001). Pre- and postcontrast CT can be used to identify the location of canine tonsils with reasonable agreement between trained observers. Discrepancy in tonsillar volume between groups of trained observers may affect RT treatment outcome.

Stereotactic radiotherapy is a highly conformal treatment option for intracranial and extracranial malignancies. Stereotactic radiotherapy utilizes specialized equipment specifically designed to avoid normal tissue while delivering ablative treatments with submillimeter precision and accuracy. Linear accelerator based stereotactic radiotherapy incorporates on-board image guidance utilizing cone beam computed tomography (CT). Many institutions lack the ability to provide image guidance with cone beam CT but delivery of highly conformal treatments with submillimeter precision and accuracy is still feasible. The purpose of this retrospective, pilot study was to describe clinical outcomes for a group of dogs with neurological disease that were treated with an stereotactic radiotherapy technique utilizing intensity modulated radiation therapy, megavoltage computed portal radiography, a bite plate, thermoplastic mold, and mask based positioning system. Twelve dogs with neurological clinical signs were included. The diagnosis of intracranial tumor was made based on advanced imaging (12/12) and confirmed via histopathology (3/12). Twelve courses of stereotactic radiotherapy, utilizing three fractions of 8.0 Gy, were delivered on alternating days. Self-resolving neurological deterioration was observed in two patients during stereotactic radiotherapy. Neurological progression free interval and median survival time were 273 days (range: 16-692 days) and 361 days (range: 25-862 days). Stereotactic radiotherapy using computed portal radiography may be a safe treatment option for dogs with intracranial tumors.