Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

Abstract

Purpose: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. Methods:We measured values of the ion recombination (k s ) and polarity (k pol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of k s were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of k s were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. Results: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of k s depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the k s value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of k pol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. Conclusion: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

Clinical commissioning of intensity‐modulated proton therapy systems: Report of AAPM Task Group 185

“…Increased chamber bias (within the chamber-bias specification range) may also reduce recombination in some cases. Finally, polarity corrections must also be considered, but they are typically negligible in proton beams for plane-parallel chambers 154,155 .…”

“…Finally, polarity corrections must also be considered, but they are typically negligible in proton beams for plane-parallel chambers. 154,155 In summary, the uncertainties associated with absorbed dose calibration of proton beams can be classified as intrinsic and practice dependent. The inherent uncertainties stem from the physical characteristics of the ionization chamber and its 60 Co calibration.…”

Dose detectors, sensors, and their applications

“…Rossomme et al . and Liszka et al . showed that, even if the dose distribution at a particular time is not uniform within the volume of the ionization chamber, the same models can be applied to scanned beams (and more generically to any nonuniform beam) if an effective ionization density or current is taken into account.…”

“…These models, which have been confirmed in numerous experimental studies, are the basis of the two-voltage method proposed by Boag and Currant. 76 Rossomme et al 72 and Liszka et al 77 showed that, even if the dose distribution at a particular time is not uniform within the volume of the ionization chamber, the same models can be applied to scanned beams (and more generically to any nonuniform beam) if an effective ionization density or current is taken into account. A similar aspect is the time-dependent structure of absorbed dose delivery in the SOBP of a clinical particle beam which can be accounted for by the use of an effective ionization current.…”

FLASH Irradiation with Proton Beams: Beam Characteristics and Their Implications for Beam Diagnostics

“…Pencil beam scanning typically employs large dose rates along the beam axis; therefore, studies about dose-rate dependent detector effects have been performed, even before FLASH became an important topic. Liszka et al [31] found small variation in recombination visible already at clinical dose rates; in particular, they found that recombination corrections estimated at an isochronous-cyclotron facility increase as a function of dose-rate. Such recombination correction variations are usually neglected in clinical practice, either because of their size (less than 0.5% in a clinical volume) or because the variations in dose rates are compensated by the accelerator system by extracting lower currents at higher energies, therefore keeping the dose rate constant at the patient.…”