Correction method for ionization chamber dosimetry in flattening filter free radiotherapy based on Monte Carlo simulation

Abstract

Background: The clinical use of flattening filter free (FFF) radiotherapy has significantly increased in recent years due to its effective enhancement of dose rates and reduction of scatter dose. A proposal has been made to adjust the incident electron angle of the accelerator to expand the application of FFF beams in areas such as large planning target volumes (PTVs). However, the inherent softening characteristics and non-uniformity of lateral dose distribution in FFF beams inevitably lead to increased dosimetry errors, especially for ionization chambers widely used in clinical practice, which may result in serious accidents during FFF radiotherapy.

Purpose: This study constructs a comprehensive Monte Carlo model that encompasses not only conventional FFF beams but also incorporates FFF beams with varying incident electron angles, to investigate dosimetry errors and correction methods in FFF radiotherapy.

Methods: We have innovatively introduced a FFF output correction factor ( $k_{Q_{FFF},Q_{WFF}}$ ) to address dosimetry errors in various ionization chambers under different incident electron angle conditions in FFF beams. The primary variations in $k_{Q_{FFF},Q_{WFF}}$ were analytically determined to result from changes in $s_{w,air}$ and the perturbation correction terms of the ionization chamber.

Results: Ionization chambers with smaller sensitive volumes typically exhibit reduced dosimetry errors. Our findings indicate that for ionization chambers with sensitive volumes ranging from 0.016 to 0.125 cm³, the dosimetry error under various FFF beam conditions consistently remains below 1.15%. This study provides crucial guidance for selecting appropriate ionization chambers in FFF radiotherapy.

Conclusion: A correlation was established between the absorbed dose to water in beams with a flattening filter (WFF) and those without (FFF), defined by the FFF output factor ( $O{F}_{Q_{FFF},Q_{WFF}}$ ). Using the proposed Monte Carlo model, the $O{F}_{Q_{FFF},Q_{WFF}}$ can be derived and applied to theoretically calculate the absorbed dose to water in FFF beams at varying incident electron angles, with a relative standard uncertainty of 0.2. This study provides a valuable reference for clinical dose measurements and crucial support for establishing dose calibration standards in FFF radiotherapy.

Keywords: FFF output factor; Monte Carlo simulation; dosimetry errors; flattening filter free beam; ionization chamber.