The Multilayer Ionization Chamber (MLIC) is an instrument in rapidly measuring the proton depth dose distribution, which is crucial for enhancing the efficiency of beam commissioning and daily quality assurance in treatment rooms.

This study aims to investigate the impact of the Water Equivalent Ratio (WER) energy dependence of various absorber materials on MLIC measurements, thereby improving the accuracy of depth dose distribution measurements.

Based on the fixed beam source parameters in the beam therapy room of Shanghai Advanced Proton Facility (SAPT), a physical model of MLIC was constructed using Monte-Carlo method. The simulation environment was validated by comparison of the measured and simulated integrated depth dose curve. The WER for three absorber materials i.e., Aluminum, PMMA, and FR-4, was calculated by simulation across different energies and thicknesses. Then, proton pencil beams of varying energies were simulated incident on MLIC, and the depth dose distribution of MLIC made from these materials was analyzed whilst the MLIC composed of water absorber was served as a reference.

Simulation results show that the energy dependence of WER significantly influences the range parameters of the depth dose distribution, which was measured by MLIC within the clinical proton radiotherapy energy spectrum, with an impact exceeding 60%, and has a lower effect on the width and the distal dose falling region length of the Bragg peak. By adopting the appropriate WER values, the disparities in depth dose distribution parameters between MLIC made from different absorber materials and that composed of water absorber can be greatly reduced. Notably, for PMMA (Polymethylmethacrylate), the range discrepancy is minimized to 0.220 mm.

The depth dose distribution measured by MLIC is notably affected by the energy dependence of WER, underscoring the importance of considering WER's energy dependence in clinical proton therapy. The study is valuable for guiding experiment tests and optimized design of MLIC.