Radiation treatment requires a precise procedure for interfraction repositioning of the patient. The
purpose of this study was to determine the accuracy of our fixation device in treatment position and to
evaluate the setup accuracy with two different methods. The positioning data of 19 canine patients with
tumors in the head region (oral, nasal, cerebral) treated with photon or proton irradiation were
included in this study. The patients were immobilized by means of an individualized fixation device.
Focus was set upon interfraction displacement with systematic and random components. In one method,
treatment position was evaluated using single plane port radiographs and megavoltage x-rays. In the
other method, two orthogonal CT-topograms were acquired to evaluate the precision of positioning of
the patient in the immobilization device. Systematic and random displacements were calculated and
presented as mean values with corresponding 95% confidence intervals. In spite of a difference between
both methods, the positioning seemed to he accurate within the expected range. It seems that a safety
margin of 3.7 mm would be enough for both methods to take into account systematic and random
position variability in the fixation device, thereby preventing geometric inaccuracies of treatment
delivery. The reported immobilization protocol provides accurate patient immobilization for photon
and conformal proton radiation therapy.