Validation of a Monte Carlo model of a NACP-02 plane-parallel ionization chamber model using electron backscatter experiments.

The accuracy of Monte Carlo (MC) simulation results relies on validating the MC models used in the calculations. In this work, a MC model for the NACP-02 plane-parallel ionization chamber was built and validated against megavoltage electron backscatter experiments using materials of water, graphite, aluminium and copper. Electron energies ranged between 6-18 MeV and the chamber's air cavity was at the depth of maximum dose, z(max). A chamber model based on manufacturer's specifications resulted in systematic discrepancies of several percents between measured and simulated backscatter factors. Tuning of the MC chamber model against backscatter factors to improve agreement increased the chamber's front window mass thickness by 35% over the reported value of 104 mg cm(-2) in the IAEA's TRS-398 absorbed dose protocol. The large increase in chamber window mass thickness was verified by measurements on a disassembled NACP-02 chamber. The new backscatter factor results based on the tuned MC NACP-02 chamber model matched the experimental results within 1-2 standard deviations. We conclude therefore that for MC simulations near z(max), tuning of the NACP-02 chamber model against experimental backscatter measurements is an acceptable method for validating the chamber model.