Dosimetric evaluation of patient-side range shifters using two materials for superficial region treatment in spot scanning proton therapy: impact of material, air gap and setup distance

This study aimed to evaluate the dosimetric characteristics of patient-side range shifters (RS) made of polymethyl methacrylate and high-density polyethylene for superficial region treatment in spot scanning proton therapy. The impact of air gap (AG) size and RS setup deviations on dose accuracy was investigated. A virtual water phantom and a simple volumetric beam arrangement were used for dose calculations in a treatment planning system employing a pencil beam algorithm with a triple Gaussian kernel. Gamma passing rates (GPRs) and point dose measurements were performed using a two-dimensional detector array and an ionization chamber, with AGs ranging from 0 to 110 mm. Results showed that GPRs exceeded 90% at depths of 9, 20 and 50 mm when the AG was within 80 mm, even under 2%/2 mm evaluation criteria. At AGs of 90-110 mm, reduced GPRs and >3% deviations in point dose measurements were observed at shallow depths. Setup deviation experiments were conducted by intentionally shifting the RS slab from the geometry used for reference measurements, and GPRs were evaluated relative to the corresponding measured reference at each original AG. The results indicated that shifts of up to ±10 mm from the reference AG had minimal dosimetric impact, with GPRs remaining above 96.8% under 1%/1 mm criteria. Maintaining the AG within 80 mm is recommended to ensure dosimetric accuracy. These findings suggest that patient-side RS systems can be a practical option in clinical scenarios where nozzle-mounted RS devices are not feasible.