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.