Fig. 4

PBS spot sizes versus proton energy from measurements at the isocenter for the open field (red), MRS with air gaps (AG) of 6, 16, and 26 cm (black), and simulation with AG of 2, 6, 10, 16, and 26 cm respectively, which were calculated with the differential Moliere formula. The AG was selected based on typical treatment conditions. UPRS can generally be placed as close to the patient's surface as 2–10 cm AG. To avoid collision between MRS and patient, the AG for brain and head and neck treatment are typically 15–20 cm. The largest AG of 35 cm was selected to represent the worst-case scenario