Optimizing Patient Positioning for Intensity Modulated Radiation Therapy in Hippocampal-Sparing Whole Brain Radiation Therapy

Abstract

To determine the optimal patient position, using an inclined headboard, for treatment planning of hippocampal-sparing whole brain radiation therapy (WBRT) using intensity modulated radiation therapy (IMRT). Sparing the hippocampus during whole brain radiation therapy offers potential neurocognitive benefits over traditional WBRT. However, previously reported IMRT plans use multiple non-coplanar beams for treatment delivery. Given the possible widespread use of IMRT for WBRT, including an upcoming RTOG trial, an optimized coplanar IMRT template for hippocampal-sparing WBRT would assist in clinical workflow and also help minimize costs. A variable angle, inclined board with the ability to mount an Aquaplast mask was utilized for patient positioning. An anthropomorphic phantom underwent CT simulation at 0, 15, 30, and 40 degree angles. Contours of the brain, lenses of the eyes and hippocampus were generated and used as goal structures for IMRT planning. IMRT goals were designed to achieve target PTV coverage while maintaining a hippocampal D100% ≤ 9 Gy and maximum hippocampal dose ≤ 16 Gy. Planning was initially performed on the 30 degree data set using a coplanar beam arrangement to deliver 30 Gy in 10 fractions. The plan was optimized and then applied to the other head angles. Dose-volume histograms were reviewed with attention paid to D100%, maximum and mean dose to the hippocampus as well as the dose to the lenses. Volumes for the hippocampus, hippocampal avoidance region and whole brain were 1.07 cm3, 12.48 cm3 and 1185.1 cm3 respectively. The hippocampal avoidance region occupied 1.05% of the whole brain planning volume. For the 30 degree head angle, a 7 field IMRT plan utilizing a coplanar beam arrangement was generated sparing the hippocampus to a maximum dose of 14.67 Gy. D100% of the hippocampus was 7.37 Gy and mean hippocampal dose was 9.34 Gy. Similar values were obtained for the 15 degree head angle. In comparison, for flat head positioning the hippocampal Dmax was 22.91 Gy with a D100% of 9.18 Gy and mean dose of 11.7 Gy. Target coverage and dose homogeneity was comparable to previously published non-coplanar IMRT plans. Compared to conventional supine positioning, the use of an inclined head board at 15 or 30 degrees allows for optimal IMRT planning for a coplanar treatment plan. Acceptable dosimetry can be obtained for coplanar hippocampal-sparing whole brain radiation therapy with the addition of an inclined head board.