Clinical Implementation of Lung SABR IMPT Using Two PBS Systems With Different Spot Sizes and Delivery Speeds

Abstract

Stereotactic Ablative Body Therapy (SABR) is an effective treatment for early-stage lung cancer. Two different proton therapy delivery systems (Proteous-plus with universal nozzle called P+ and Proteus-one called P1) were employed to test the spot size difference, plan dosimetric quality, and delivery time needed for practical and effective clinical treatment.Total of 9 lung patients with 10 tumors were selected for this study. Seven targets were prescribed 60 Gy in 10 fractions; three patients were 48 Gy in 4 fractions. A treatment planning system was used to generate treatment plans for each nozzle type with exact same optimization objectives. P1 plans used 4cm range-shifter placed at 45cm from isocenter to create large in-air beam spots to mimic P+ system. The spot sizes were compared to the P+ in-air spots without range shifter. P+ plans used 7cm range-shifter placed very close to patients, i.e., 4cm away, to minimize spot size degradation. Target coverage was optimized and normalized to prescription dose cover 100-percent of GTV and 90-percent covers PTV. Dose to the OARs were compared using QUANTEC metrics. Each plan was split into three subsets to perform volumetric dose-repainting to reduce interplay effect. The plan beam-on times of both systems were obtained to evaluate treatment delivery efficiency and impact of patient comfort.The in-air beam spots were comparable between the P+ without and P1 with range shifter. All PTVs were covered by 90-percent prescription dose. OAR doses were comparable and within the QUANTEC limits. The beam-on time of P+ plans are 3 to 6 time of those of P1 plans. When combined with patient setup and image guidance, patient discomfort could lead to higher uncertainty in P+ treatment.P1 system with 45cm away from isocenter can effectively simulate a large spot PBS system, P+. Lung SABR IMPT plan qualities between the two systems were dosimetrically comparable. Volumetric dose-repainting were applicable to both systems to reduce interplay effect. However, the plan beam-on and delivery time of P1 system were significantly shorter than that of P+. Therefore, potential less patient movement during treatment leads to less dose uncertainties to patients.