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

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.

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.

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.

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.