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

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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). 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 0.2cm. 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. 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. 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 0.2cm in WET.