Technical Note: Quality assurance of proton central axis pencil-beam spread-out Bragg peak using large-diameter multilayer ionization chambers.

PURPOSE: The aim of this work is to describe a method of machine quality assurance (QA) by measuring proton spread-out Bragg peak constructed by the integrated depth dose via a large-diameter (12-cm) multilayer ionization chamber (LD-MLIC). METHODS: Two types of contours are used to create the nominal plan. The final nominal plan is composed of mixed-energy proton pencil-beam spots located close to the central axis. The integrated depth dose (IDD) curve contains a flat SOBP region. The LD-MLIC-measured IDD was compared to the IDD curve exported from the treatment planning system (TPS). In addition, three plans with intentionally modified energy layers to simulate wrong-delivered energy layers were created and measured by the LD-MLIC. The water equivalent thickness (WET) difference between the inserted and replaced energies was 0.2 cm. Six weeks of measurements were analyzed. A low-pass filter was introduced to mitigate the high-frequency noise in the IDD signal ratios. The filtered IDD signal ratios between the modified plans in different weeks and the baseline were used to check the energy accuracy. RESULTS: The differences between the LD-MLIC-measured and TPS-exported IDDs of the nominal plan were within 2% in most parts of the curve. Bumps/dips (~1%) were noted in the filtered IDD ratio between the modified plans and the baseline. CONCLUSIONS: The LD-MLIC can be used to check the accuracy of multiple energies Bragg peak locations quickly in proton machine QA. The LD-MLIC was sensitive in identifying an erroneous energy with 0.2 cm in WET.