ESTRO 2023 - Abstract Book

S1714

Digital Posters

ESTRO 2023

1 Cleveland Clinic, Radiation Oncology, Cleveland, USA; 2 Advanced Oncotherapy, Application of Detectors and Accelerators to Medicine, Meyrin, Switzerland Purpose or Objective To compare plan quality among Gamma Knife plans and aperture-based Intensity-Modulated Proton Therapy (IMPT) plans for cyclotron-generated proton beams (CPB), linear accelerator proton beams (LPB), and linear accelerator proton minibeams (LPMB). Materials and Methods Treatments for fifty-eight brain lesions from twenty patients were planned using aperture-based IMPT for CPBs, LPBs, LPMBs; Gamma Knife photon-based plans were also generated for each. All proton plans were optimized with the multi- field optimization (MFO) technique, using the Monte Carlo algorithm. For all plans (proton and Gamma Knife), dose coverage to each lesion was set to 99% of the GTV receiving the prescription (Rx) dose. For the proton plans, robustness was evaluated as ± 2 mm setup uncertainty and ± 2% range uncertainty using V100%Rx > 95% of the GTV. An aperture margin of 3 mm from the GTV was utilized to incorporate the 2 mm robustness. Conformity index (CI), gradient index (GI), V12Gy, V4.5Gy, and mean dose to the brain were compared across all plan types. Conformity index for IMPT plans was defined as the 100% isodose volume covering the GTV divided by the volume of the GTV expanded by 2 mm isotropically to account for robustness. Conformity for Gamma Knife plans was evaluated using the RTOG conformity index, as the 100% isodose volume divided by target (GTV) volume. The statistical significance was evaluated between dosimetric quantities for all modalities using the Wilcoxon signed rank test. Results Aperture-based IMPT plans showed improvement from Gamma Knife plans for both CI and GI, with average CIs of 1.01 ± 0.4, 0.97 ± 0.4, and 0.89 ± 0.3, and average GIs of 3.6 ± 1.4, 2.98 ± 1.0 and 2.88 ± 1.0, for CPB, LPB, and LPMB plans, respectively. In comparison, the mean CI and GI for Gamma Knife were 2.1 ± 0.8 and 3.3 ± 1.1, respectively. Table 1 displays the dosimetric indices across all plan types. CPBs reduced the mean brain dose by 62%, LPBs reduced the mean brain dose by 68%, and LPMBs reduced the mean brain dose by 72%, compared to Gamma Knife. The V4.5Gy and mean brain dose were most improved when using aperture-based IMPT, compared to Gamma Knife plans. Figure 1 a and b show the trend in decreasing mean brain dose from Gamma Knife plans down to proton minibeam plans and corresponding isodose distributions, respectively. While all proton plans had V100%Rx of the GTV > 99% in the nominal scenario, these proton plans achieved V100%Rx > 95% in the worst-case scenario. All results showed statistical significance between plan modalities, with P < 0.05.