Increased Efficiency of Range Verification in Routine QA for Pencil-Beam Scanning Proton Therapy

Quality assurance (QA) is required when performing pencil-beam scanning proton therapy, but the efficiency of QA is degraded in proportion to the energy of the protons. We developed a method to assess the preferred energy range and distal fall-off by combining multiple Bragg peaks to increase the efficiency of QA. Beams of 70, 110, 150, 190, and 230 MeV for exposure were planned using a treatment planning system. The Bragg curves for therapeutic proton beams were modeled using three different fitting function models, allowing the feasibility of a simple modeling of the Bragg curve to be investigated. The planned beams were exposed and measured using a multi-layered ionization chamber. Software developed using a Python tool could detect five Bragg peaks from the integrated curves that were fitted based on polynomial, cubic spline and Landau distributions. This software could calculate the range and distal fall-off of the five fitted peaks. For the verification of the accuracy of this method, the calculated results were compared with the range and distal falloff obtained by exposing and analyzing five single-energy beams individually. Comparisons of the Bragg peaks for the five energies exposed individually with the results obtained by exposing them all at once showed that the ranges of the energy beams when using the polynomial fitting and the cubic spline modes were 0.16 mm and 0.10 mm longer, respectively, while the distal fall-offs were 0.14 mm and 0.06 mm shorter, respectively. When using the Landau distribution fitting, the range was 0.06 mm longer and the distal fall-off was 0.04 mm shorter. Analyses of the ranges and distal fall-offs of the five energy beams exposed at once with single-beam loading by using the method developed in this study showed no significant differences from the results obtained by exposing the energy beams individually. Thus, range verification QA by using the proposed method is not only suitable for single-proton beams with multiple energies but also reduces the measurement time.