The model to correct for the ion recombination of micro-thin cylindrical ionization chamber in high dose-rate and frequency pulses X-ray beams

Accurate dosimetry is essential for ultrahigh dose-rate (UHDR, >40 Gy/s) irradiations in clinical radiotherapy (RT). Herein, we introduced a prototype of a micro-thin cylindrical ionization chamber (MC-IC) featuring an electrode distance of 0.2 mm, specifically designed for these beams. This study aimed to investigate the ion collection efficiency (ICE) of the MC-IC in high dose-rate and frequency (HDR&F) pulse X-ray beams. A detailed comparison was made among an empirical logistic model, the two-voltage analysis (TVA) method, and the Boag model. As anticipated, the MC-IC successfully measured a mean dose rate of up to 154.5 Gy/s with an ICE of 98.6 %. We found that the empirical logistic model is well-suited for the MC-IC, whereas the TVA method and Boag model exhibit an underestimation of ICE in HDR&F beams. Therefore, Boag Model 3 with a free-electron fraction of 0.5 < P < 0.6 and a modified TVA equation with a free-electron fraction of P = 0.6 must be considered. For the TVA method, a voltage ratio <= 2 is recommended. In conclusion, this study complemented and refined a dosimetric methodology for evaluating the collection efficiency correction factor for special ionization chambers.