Beam Delivery System

The Siemens ion beam delivery system used in this study has a synchrotron for acceleration and a modulated scanning beam technique paired with energy stacking for beam delivery. The accelerator can produce proton and carbon ion beams with energies ranging from 48.08 to 221.07 MeV/n and 86.22 to 430.12 MeV/n, respectively. Each ion species can be accelerated and extracted at one of the 296 different preprogrammed energy levels, which correspond to an approximately 1 mm step of range in water. For each energy level, there are five different spot sizes. The spot sizes at isocenter in air are 8.10 to 32.65 mm and 3.38 to 13.53 mm for proton and carbon ion beams, respectively. Higher energy has smaller spot size. The modulated scanning technique makes the beam remain at each spot until the requested monitor units is reached and then swiftly moves the beam to the next spot without turning off the beam. The ion beam delivery system has a dynamic intensity control system (DIC), which computes and adapts the extraction rate during the delivery of ions at each spot to make efficient and accurate extractions. Beam delivery is monitored by the beam application and monitoring system (BAMS). Figure 1 shows the BAMS diagram. Beam parameters measured by the BAMS are recorded in the log file. The BAMS in each treatment room consists of three transmission ionization chambers and two multiwire proportional chambers. The ionization chambers are used to monitor the number of ions delivered at each spot. Multiwire proportional chambers are used to monitor the spot position and size. The spot positions and spot size (horizontal and vertical widths) are measured every 250 μs by multiwire proportional counters. The flux is sampled by a transmission parallel plate ionization chamber at 1-μs time intervals, which can quickly turn the beam to the next spot when reaching the prescribed particle numbers in one spot.

Diagram of the beam application and monitoring system (BAMS) which consists of three ion chambers (IC and IM) and two multiware proportional chambers (MW). The spot size, spot position, and particle number in each spot is measured by the BAMS and are recorded in a log file.