Evaluation of ion recombination and polarity effect on photon depth dose measurements using mini- and micro-ion chamber

Abstract

Purpose: To investigate the effect of ion recombination ( $k_s$ ) and polarity ( $k_{pol}$ ) correction factors on percentage depth dose (PDD) curves for three ion chambers, using flat and flattening filter free (FFF) beams, across different broad field sizes. A method to assess these effects and their corresponding corrections is proposed.

Methods: $k_s$ and $k_{pol}$ were evaluated following the IAEA TRS-398 protocol for three ion chambers: PTW Semiflex-3D-31021, PinPoint-3D-31022, and Semiflex-31010. PDD measurements were acquired from d_max to 32 cm depth at four voltages (± 400 V or ± 300 V, and ± 100 V) for field sizes 4 × 4, 10 × 10, 20 × 20, and 40 × 40 cm². The $k_s$ values were computed after the $k_{pol}$ correction at each applied voltage. This study aimed to assess the variation of the factors along scanning depths for different field sizes, to evaluate the need for correcting the scanning data.

Results: $k_{pol}$ was independent of depth and field size for Semiflex-3D, while it presented an increasing value with depth for the PinPoint-3D. $k_s$ increased with dose rate, i.e., decreased with depth. The variation of this perturbation effect over the PDD range was about 0.1% for flat beams with all three ion chambers. With FFF beams, it was around 0.3% with PinPoint-3D, and 0.8% for 10FFF with Semiflex-3D. A second-order polynomial fit can be determined to directly correct the raw data scanned along the beam axis for both $k_s$ and $k_{pol}$ .

Conclusion: $k_s$ can significantly affect the PDD scanning measurements in FFF beams acquired with Semiflex-3D. An error close to 1% at large depths could be present, meaning an error of less than 0.3% relative to the dose at the depth of d_max.

Keywords: FFF; Ion recombination factor; photon beams; polarity effect.