Dosimetric verification on HIMM at the Wuwei heavy ion therapy center using anthropomorphic phantoms

Heavy Ion Medical Machine (HIMM) at Wuwei Heavy Ion Therapy Center has been officially approved. The aim of this study was to demonstrate the dosimetric stability and safety of HIMM prior to subsequent clinical trials. Dosimetric verification of treatment plans was conducted using anthropomorphic phantoms in a uniformly scanned beam delivery mode. Both head-and-neck and thoracoabdominal regions of anthropomorphic phantoms dedicatedly inserted with a Farmer-type ionization chamber were scanned under a 64-slice CT scanner. Planning target volumes (PTV) were delineated according to the two sets of acquired CT images. The sensitive volume of the chamber was located in the center, rising edge, plateau and falling edge of PTV, respectively. Then treatment plans were designed for the target volumes using a carbon-ion radiotherapy planning system. Dose measurements were conducted following the IAEA guidelines. Virtual respiratory signals were generated to simulate respiratory conditions, and gating irradiation was conducted on the thoracoabdominal phantom. Dose measurements were performed three times at each point. For the dosimetric verification of both the head-and-neck and thoracoabdominal phantoms, the deviations between planned and measured doses were confirmed to be within acceptable limits. For respiratory gating irradiation, the deviation was 0.32% in the center of PTV. The irradiation time was increased approximately twofold as compared to the non-gated situation. Intraclass correlation coefficient (ICC) value of the measured dose was 1.000. The results met the requirements. The passive beam delivery mode of HIMM demonstrated satisfactory dosimetric performance, making it suitable for the upcoming clinical trials.