Baselines of daily quality assurance for proton pencil beam scanning using the Sphinx Compact 

Quality assurance (QA) is a systematic process used to determine whether the product meets specified requirements. The American Association of Physicists in Medicine (AAPM) published a comprehensive proton therapy machine QA in TG-224 to provide confidence in use. King Chulalongkorn Memorial Hospital (KCMH) recently operated the first proton therapy center in Thailand, and the data of daily QA are not available or established. This study aims to apply the control chart for establishing the baselines of daily QA for the proton pencil beam scanning system using the Sphinx Compact detector. The Sphinx Compact was used to perform the proton daily QA test for a total of 93 days and analyzed by myQA software according to the AAPM TG-224 recommendation. The baselines obtained by a control chart were divided into 5 subjects: 1) the depth verification including the distal and proximal depth, distal falloff, and peak width of 100 MeV, 150 MeV, and 200 MeV, 2) spot characteristics including position, sigma, skewness, and intensity of 70 MeV, 100 MeV, 125 MeV, 150 MeV, 175 MeV, and 200 MeV, 3) x-ray/proton coincidence, 4) homogeneity and 5) dose output. The upper and lower control limits (UCL and LCL) of the control chart were calculated from 50 consecutive data in each test. All the results of control limits were found to be less than the limits recommended by AAPM TG-224. The highest limits of distal depth, distal fall-off, proximal depth, and peak width were ± 0.5 mm in all tasks from 200 MeV. For spot characteristics, the highest limits of spot position, spot sigma, spot skewness, and intensity were ± 1.2 mm, ± 2.9 mm, ± 0.621, and ± 1.8 % respectively. For x-ray/proton coincidence, the limits were found to be ± 0.4 mm and ± 0.2 mm in X and Y directions, respectively. The limits of uniform field homogeneity and dose output consistency were ± 0.7 % and ± 0.010 nC, respectively. In conclusion, the control charts obtained by Sphinx Compact can be applied to set the baselines of daily QA data for proton pencil beam scanning at Her Royal Highness Princess Maha Chakri Sirindhorn Proton Therapy Center, KCMH.