Room: ePoster Forums
Purpose: To determine the ideal photon energy for whole brain radiotherapy in order to have adequate coverage while also maintaining homogeneity and decreasing both maximum point dose and dose to the orbits.
Methods: Ten patients receiving whole brain radiation, 30 Gy in 10 fractions, were planned using 6MV, 10MV and 18MV beams. The treatment plans were created using Pinnacle 16.2 and were each normalized such that 98% of the brain volume received 100% of the prescription dose. The conventional 3D setup of two lateral-opposing brain fields was used for each patient, and equal coverage was obtained for each energy by modifying the normalization percentage at the calculation point which was located approximately mid-brain.
Results: The following parameters were considered for the analysis: max point dose, D2 (the dose to 2% of treatment volume), D98 (the dose to 98% of treatment volume), homogeneity index, orbital dose and percentage of dose normalization. The homogeneity index serves as a scoring tool to analyze uniformity of dose with 1.0 being ideal and for less homogeneous plans, a score greater than 1.0. Based on the results, as energy increases, homogeneity increases while max point dose decreases. The orbital doses were also considered and averaged for each treatment plan. We found the dose to the orbits to decrease with increasing energy.
Conclusion: After both quantitative and qualitative analysis, 18MV beams provide adequate coverage for a whole brain treatment plan while limiting orbital dose and unnecessary hotspots in the brain. The homogeneity of each plan was superior as the beam energy increased. In past literature, there was concern with achieving adequate peripheral and meningeal dose using 10MV of higher energy beams. However, we have demonstrated that with modern linac technology, it is possible to achieve superior coverage and gain the benefits of higher energy plans.