Proton beam spot size and position measurements using a multi-strip ionization chamber.

PURPOSE: To develop and characterize a large-area multi-strip ionization chamber (MSIC) for efficient measurement of proton beam spot size and position at a synchrotron-based proton therapy facility. METHODS AND MATERIALS: A 420 mm x 320 mm MSIC was designed with 240 vertical strips and 180 horizontal strips at 1.75 mm pitch. The MSIC was characterized by irradiating a grid of proton spots across 17 energies from 73.5 MeV to 235 MeV and comparing to simultaneous measurements made with a reference Gafchromic EBT3 film. Beam profiles, spot sizes, and positions were analyzed. Short term measurement stability and sensitivity were evaluated. RESULTS: Excellent agreement was demonstrated between the MSIC and EBT3 film for both spot size and position measurements. Spot sizes agreed within ± 0.18 mm for all energies tested. Measured beam spot positions agreed within ± 0.17 mm. The detector showed good short term measurement stability and low noise performance. CONCLUSION: The large-area MSIC enables efficient and accurate proton beam spot characterization across the clinical energy range. The results indicate the MSIC is suitable for pencil beam scanning proton therapy commissioning and quality assurance applications requiring fast spot size and position quantification.

The magnet sorting of APTRON.

The magnetic field error caused by imperfection during the manufacturing process of magnets is inevitable. Therefore, the field error of both dipole and quadrupole magnets and their alignment errors will cause the synchrotron closed orbit distortion. In order to minimize the influence of the beam, a sorting method is applied to both dipole and quadrupole magnets with integral consideration of different excitation currents, covering both low and high field situations. The commissioning result shows that we can achieve the designed beam intensity.

Scanned Proton Beam Performance and Calibration of the Shanghai Advanced Proton Therapy Facility.

The Shanghai Advanced Proton Therapy facility (SAPT) is a hospital-based facility that began construction in December of 2014 with commissioning of the first scanned proton beam line starting in October of 2017. Proton beams are extracted from a synchrotron accelerator with energies between 70 and 235 MeV. Beam delivery uses the modulated scanning and energy stacking techniques to produce a maximal scanning area of 40 × 30 cm2 at the iso-center. Prior to clinical use, the beam delivery system was characterized and calibrated following the guidelines of the IEC 62667 medical electronic equipment standard including the spot size in air, spot position, depth dose distributions, and lateral dose profiles, as well as the beam monitor calibrations following the IAEA TRS-398 recommendations with small differences. •The measured dosimetric results showed that the full width at half maximum (FWHM) for the beam spot size in air varied approximately from 6 mm to 13 mm. The dose fall-off (DDF) derived from the measured depth dose in water varied from 4.7 mm at 235 MeV to 0.7 mm at 70 MeV. The homogeneity of the scanned field was better than 2% for various energies as expected.•Furthermore, the beam reproducibility and proportionality delivery accuracy was also stable with the results better than 0.1% and 1% respectively. Finally, the dose monitor calibration factor, its reproducibility and stability were tested. Reproducibility tests exhibited a standard deviation (SD) result of less than 1% during the test period.•All the measured dosimetric parameters showed that the design specifications were well achieved and the results are suitable for being used as a part of the clinical commissioning and quality assurance program for treating patients.