Possibility of Interchanging Patients for Beam-Matched Linear Accelerators from the Same Vendor

The purpose of this study is to investigate the possibility of interchanging patients on beam-matched linear accelerators (linacs), by evaluating the dosimetric variation and the dose delivery accuracy of volumetric modulated arc therapy (VMAT). Two VitalBeam linacs installed recently in our center were included in this study. For dosimetric data comparison, beam data, such as percentage depth doses (PDDs), beam profiles, output factors, multi-leaf collimator (MLC) leaf transmission factors, and the dosimetric leaf gap (DLG), for 6-MV and 10-MV beams were measured on both linacs. Sixty patients (20 head and neck, 20 lung, and 20 prostate cancers) were selected to evaluate the dosimetric parameters and the delivery accuracy of the VMAT plans created for various treatment sites. All the VMAT plans were generated using the Eclipse treatment planning system (TPS) with the same optimizer algorithm. The dose differences between two beam-matched linacs for planning target volumes (PTVs) and organs at risk (OARs) were recorded and analyzed. Patient-specific dosimetric quality assurance (DQA) using the Electronic Portal Image Device (EPID) were performed, and the measured planar doses were compared with those calculated by using the TPS though absolute dose gamma evaluation using 2%/2 mm and 2%/3 mm criteria. The differences in all dosimetric beam data between the two beam-matched linacs were all within 1% for the 6-MV and the 10-MV beams. For all 60 cases of three treatment sites, no notable differences were observed in the dosimetric comparison and the DVH variation between the beam-matched linacs. The average differences in dosimetric parameters of PTV and OARs were all within 1%. Furthermore, all passing rates of measured doses on the beam-matched linacs were higher than 93% and 97% for the 2%/2 mm and the 2%/3 mm criteria, respectively. Our overall dosimetric variation in beam data and VMAT plans remains within the tolerance of clinical acceptability. Pretreatment verification results were also clinically acceptable, showing good agreement with the calculated doses. Therefore, this study demonstrated the possibility of swapping patients in a VMAT plan without any modification and re-planning between two beam-matched linacs.