Ion recombination correction in reference dosimetry for pencil beam scanned proton beams

The 2024 IAEA TRS-398 revision updated recommendations for reference dosimetry and ion recombination corrections in pencil beam scanned (PBS) proton beams.

This study evaluates the revised ion recombination methods for monoenergetic synchrotron-based PBS proton system across different energies, monitor units (MU), and ionization chamber types.

Reference-field measurements were performed using a synchrotron system at 70.2, 150.2, and 228.7 MeV and at 6, 50, and 200 MU. Charge-collection data were acquired using PTW Farmer and Advanced Markus chambers across 20-400 V. Ion recombination correction factors ( $k_s$ ) were determined using the Jaffé plot extrapolation method and the TRS-398 two-voltage method (TVM) under different time structure assumptions. Charge multiplication in the chamber was addressed using both low voltage linear fitting and a semiempirical exponential model.

For low energy, low MU fields, and TVM yielded $k_s$ values within ∼1% of the Jaffé extrapolation. For high-energy, high-MU fields, maximum differences of 6.25% (Farmer) and 1.62% (Advanced Markus) were observed. The synchrotron beam exhibited energy, MU, and chamber dependent time structure behavior, producing pulsed-like or continuous-like characteristics. Misclassification of the time structure resulted in additional $k_s$ deviations of up to 2.49% (Farmer) and 0.59% (Advanced Markus). Charge multiplication was observed in the Advanced Markus chamber at voltages > 150 V. The exponential fitting successfully modeled this response and produced $k_s$ values agreeing with low voltage fits within 1.5%, while avoiding subjective voltage cutoff selection.

The revised TRS-398 provides accurate ion recombination corrections for monoenergetic PBS fields at low energies and low MU. However, accuracy of ion recombination correction decreases at higher energies and MU, particularly when the time structure was ambiguous or chamber dependent. Charge multiplication in small volume chambers presents an additional source of uncertainty not fully addressed by TRS-398. Incorporating charge multiplication fitting methods may improve the robustness of reference dosimetry in synchrotron-based PBS proton therapy.