Validation of new bite block-type head-immobilization devices for radiotherapy in dogs

An ideal head-immobilization method provides a high level of accuracy and reproducibility in the immobilization. Various head-immobilization methods for radiotherapy have been published and are excellent in terms of accuracy; however, these methods are complicated to use, and labor intensive. The present study describes two new bite block-type head-immobilization devices designed for higher stability and lower vertical variation. The device designed in our previous study (the bite block-type head-immobilization device; Device A) was modified by making a groove on the top the horizontal plate (Device B) for a stable ventral-dorsal position, or beneath the horizontal plate (Device C) for a stable dorsal-ventral position. The three devices were objectively compared with respect to setup time, and accuracy of the computed tomography scan images by two authors independently. Five male healthy beagles were used in this study. For each device, the setup time and the variation in the coordinates were measured five times for each dog. The mean setup times for Devices A, B, and C were 3.3, 1.5, and 2.4 min, respectively, showing the groove modifications were able to reduce the setup time (in device B, by at least 50%). Moreover, three-dimensional analysis of the computed tomography images revealed that the measurement variability of Device A (1.6 ± 1.0 mm) was significantly higher than that of Device C (0.7 ± 0.4 mm; P < 0.001). Collectively, our results show that use of a bite block-type head-immobilization device with a groove improves the setup time and head-immobilization accuracy.