Investigation of ion recombination and polarity effects in high dose rate electron beams

Accurate dosimetry is essential for enhancing the effectiveness and safety of radiotherapy treatment. High dose rate electron (HDRE) beams, a dose rate up to ten times higher at the measurement point than standard electron beam at the same Monitor Unit (MU). The increased ionization density raises concern about ion recombination effects, which may affect the reliability of ionization chamber readings.

This study aims to evaluate the effects of ion recombination ( $k_s$ ) and polarity ( $k_{pol}$ ) across various dosimetry protocols in order to determine the most suitable approach for HDRE settings.

Measurements were carried out using Elekta VersaHD, linear accelerator at 6 MeV and 10 MeV both in standard and HDRE modes using three ionization chambers: IBA PPC40, FC65-P and CC13. Data were collected at a source-to-surface distance (SSD) of 100 cm, referring to the standard dosimetry protocols TRS-398 and AAPM TG-51, along with a modified method. Modified method using beam quality conversion factor ( $k_Q^{\text{'}}$ ) derived from Monte Carlo simulation and specific adjustment was applied to PPC40, where the Effective Point of Measurement was determined to be 1.39 mm from the window surface.

Based on three dosimetry protocols, all ionization chambers showed improved performance, with the $k_s$ values decreasing by approximately 0.4% to 3% as the operational voltage increased from 100 to 400 V, indicating enhanced charge collection and less recombination effects. In contrast, all of $k_{pol}$ values remained remarkably stable, ranging from 0.9962 to 1.0001. This study evaluated ion recombination and polarity effects in HDRE beams using plane parallel and cylindrical chambers.

Based on data variations, we recommend elevated operating voltage (400 V) than typical voltage (300 V) to reduce ion recombination in HDRE beams. Furthermore, modified method can be alternative to measure the accuracy of charge readings for absolute dosimetry.