WE‐E‐AUD B‐03: Analysis of Dose Perturbation Factors for An NACP‐02 Parallel‐Plate Ionization Chamber in Clinical Electron Beams

Abstract

Purpose: To investigate the dose perturbation factors pwall, pcav, pQ in water of an NACP‐02 parallel‐plate ionization chamber in clinical electron beams (4–18 MeV). These perturbations factors are assumed unity in current dosimetry protocols, but recent publications have cast doubts on this. Method and Materials: An EGSnrc Monte Carlo model was developed for the NACP‐02 parallel‐plate ionization chamber. Perturbation factors for the presence of the non‐water wall pwall, the presence of an air cavity pcav and the total perturbation factor pQ were calculated in water. The influence of the front and back walls were investigated. The influence of electron beam energy and depth in water were investigated. Results: pwall is significantly different from unity for low energy electron beams, and decreases with energy. pcav is close to unity for all beam energies. Both pwall and pcav increase significantly with depth in water, leading to an increasing pQ, which reaches a value of 1.23 at a depth of R50 for 4 MeV electrons. This was, however, found to depend strongly on whether scaling of the non‐water equivalent front wall of the chamber was performed; pQ decreased from 1.23 to 1.02 for 4 MeV electrons at R50 when scaling is done. At the reference depth zref, little effect of the window scaling was noted. The back and front wall of the ion chamber were found to have opposite influences on pwall. The effect of in‐scattering and track obliquity on the depth dependence of pcav was investigated. Conclusion: The findings of an increased pQ with depth in water could have important consequences for clinical depth dose measurements. However, the issue of front window scaling needs further study, and recommendations need to be issued. The behaviour of pcav and pwall with depth and energy were explained. Studies like these may allow improved chamber design.