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

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.