Mapping initial and general recombination in scanning proton pencil beams.

The ion recombination is examined in parallel-plate ionization chambers in scanning proton beams at the Danish Centre for Particle Therapy and the Skandion Clinic. The recombination correction factorksis investigated for clinically relevant energies between 70 MeV and 224 MeV for dose rates below 400 Gy/min in air. The Boutillon formalism is used to separate the initial and general recombination. The general recombination is compared to predictions from the numerical recombination code IonTracks and the initial recombination to the Jaffe theory.ksis furthermore calculated with the two-voltage method (TVM) and extrapolation approaches, in particular the recently proposed three-voltage (3VL) method.. The TVM is in agreement with the Boutillon method and IonTracks for dose rates above 100 Gy/min. However, the TVM calculatedksis closer related to the Jaffe theory for initial recombination for lower dose rate, indicating a limited application in scanning light ion beams. The 3VL is in turn found to generally be in agreement with Boutillon's method. The recombination is mapped as a function of the dose rate and proton energy at the two centres using the Boutillon formalism: the initial recombination parameter was found to beA=(0.10+/-0.01) V at DCPT andA=(0.22+/-0.13) V at Skandion, which is in better agreement with the Jaffe theory for initial recombination than previously reported values. The general recombination parameter was estimated tom(2)= (4.7+/-0.1)x10(3)V(2)nA(-1)cm(-1)andm(2)= (7.2+/-0.1)x10(3)V(2)nA(-1)cm(-1). Furthermore, the numerical algorithm IonTracks is demonstrated to correctly predict the initial recombination at low dose rates and general recombination at high dose rates.