Comparison of PSPT, IMPT and photon SBRT for tumors near critical structures. A. PSPT (PS) or clinical photon (Clinical) SBRT for lesion near the bronchial tree and heart. B. IMPT or clinical photon (Clinical) SBRT for lesion near the bronchial tree, aorta, esophagus, and heart. (a) Color wash dose distributions shown with corresponding scale; maximum determined by plan with lower PTVmax. In the IMPT images, the gray zone seen inside the PTV represents where the IMPT plan had a dose higher than the scale maximum. Abbreviations: BT = bronchial tree; E = esophagus; SC = spinal cord; A = aorta; H = heart; PV = pulmonary vessel; PTV = plan target volume; GTV = gross target volume. (b) Dose-volume histograms. Dotted lines = PSPT or IMPT plan; solid lines = photon SBRT plan.