Initial Experience With A New Generation Of Pencil Beam Scanning Gantry For The Treatment Of Cancer Patients

The new generation proton spot scanning gantry was developed in-house and clinically commissioned in 2013. It is a compact gantry delivering the dose by dynamic energy changes between 70 – 230 MeV and magnetic scanning of very narrow pencil beams in two orthogonal directions. As of November 2014, 22 patients have been treated. The purpose of this study was to analyze the results of all patient specific QA measurement, evaluate the treatment delivery accuracy and reflect on the first year of its clinical use. All the plans were created with the in-house developed treatment planning system which uses analytical ray casting model for dose calculation. The plans are a combination of single field uniform dose (SFUD) and intensity modulated proton therapy (IMPT). Every treatment plan is translated in the machine data file containing the information about selected Bragg peaks, their weight and position. Corrections for nuclear interaction as well as spot delivery precision are based on measurements. Final dosimetric verification of each machine file consists of 2D measurement performed with a commercial ionization chamber array mounted on an automatically controlled water column such that measurements at different depths can be performed as required. The measured profiles are directly sent to the treatment planning system where they are compared with the predicted doses. A computer algorithm-based routine, launched directly from the TPS, was developed to perform 2D γ analysis. Global γ analysis with two sets of criteria was performed: 3%, 3mm (3-3); 5%, 1mm (5-1). The dose threshold was 10% of prescribed dose. One hundred fifty (46 series) fields delivered clinically to 22 patients were analyzed. From that 109 fields (35 series; 76%) were SFUD and 41 fields IMPT (11 series; 24%). The γ analysis has shown that 89% and 19% of all fields has more than 90% of all pixel with γ <=1 for 3-3 and 5-1, respectively. Looking at SFUD and IMPT separately, the results were 89% and 88% for 3-3 and 24% and 7% for 5-1, respectively. Allowing a 1mm shift correction for misplacement of the verification device the results improved to 99%, 99% and 100% for 3-3 and 81%, 85% and 68% for 5-1 for all fields, only SFUD and IMPT, respectively. Verifications have been performed to evaluate the precision of dose delivery. The presented results show that the delivery is reliable and accurate. IMPT results are slightly worse due to the steep dose gradients which are sensitive to any set-up error. Our current experience indicates that the patient treatment delivery was safe and the patient through put can increase in coming years.