Utility of a patient-specific aperture collimator and multi-leaf collimator in line scanning proton therapy

Scanning proton therapy offers advantages over conventional photon therapy due to the Bragg-peak effect and superior dosimetric precision. Compared with passive methods, however, scanning proton therapy tends to exhibit a wider penumbra in the low-energy range. To address this issue, the present study evaluated the effectiveness of a patient-specific aperture collimator (AC) and multi-leaf collimator (MLC) for the line proton scanning treatment system at the Samsung Proton Therapy Center in Korea. The penumbra reduction efficiency of the collimation devices and the neutron dose was assessed through Monte Carlo (MC) simulations. The clinical effectiveness was evaluated dosimetrically and by assessing quality assurance. Through the MC calculation, the AC displayed the highest efficiency in penumbra reduction (43.75% of no collimator’s penumbra width at a depth of 10 cm with a 150 MeV proton beam), while the MLC also presented a comparable effect (56.25%). The neutron dose was compared at varying distances from the field edge. The neutron dose was higher in the order of AC(+ 11.96%p), MLC(+ 2.61%p), and no collimator at the 10 cm distance from field edge. For the dosimetric study, treatment plans for the nasal cavity, prostate, liver, lungs, and breasts were generated by the treatment planning system using the MC algorithm. Three treatment plans were developed for each of these five sites: line scanning without any collimation, line scanning with patient-specific ACs, and line scanning with MLCs. These plans were then compared dosimetrically, and gamma pass-rates were measured at various depths. These multifaceted comparisons showed that the use of an AC optimized the OAR sparing effect, with an MLC having similar effects, while maintaining the same target coverage and producing less neutron dose. All measurement results demonstrated a gamma pass-rate (2%, 2 mm) of over 90%, indicating the feasibility of implementing these techniques in actual clinical practice.