Room: AAPM ePoster Library
Purpose: In whole-brain irradiation, the parotid gland (PG) is recognized as one of the organs at risk; however, the beam delivery from the left and right sides cannot reduce the PG dose. It was reported that the four-field box technique using the tilting device for the head would be able to reduce the PG dose by excluding it from the radiation field. The purpose of this study was to determine the appropriate head tilt angle to reduce the PG dose in the four-field box technique.
Methods: Ten patients who previously had whole-brain irradiation were selected. First, the conventional bilateral beams were set so that the beamline of the anterior side was parallel to the line connecting the left and right eye sockets. Additionally, to simulate the head tilt, a beam from the anterior direction was added with the couch angle of 90-degree. The beam was selected at an angle that could separate the lens from the irradiation field, and the beam from the posterior direction was set on its opposite side. The beam angle from anterior direction subtracted the real orbitomeatal plane angle (OMPA) to the vertical of the couch was defined as the virtual OMPA (direction of increase: direction in which the patient’s jaw is pulled).
Results: The average dose to the PGs increased with the increase of the virtual OMPA (Spearman's rank correlation, r = 0.59, p <0.001). Conversely, the maximum dose to the lens decreased with increasing the virtual OMPA (r = -0.63, p <0.001). The virtual OMPA that reduced the lens dose to less than 10-Gy with a 95% confidence interval was approximately 20-degree, while the average dose to the PGs was approximately 8.1-Gy (30% reduction from the conventional bilateral technique).
Conclusion: The orbitomeatal angle of 20-degree can effectively reduce the parotid gland dose.