Use of a novel two-dimensional ionization chamber array for pencil beam scanning proton therapy beam quality assurance

The need to accurately and efficiently verify both output and dose profiles creates significant challenges in quality assurance of pencil beam scanning (PBS) proton delivery. A system for PBS QA has been developed that combines a new two-dimensional ionization chamber array in a waterproof housing that is scanned in a water phantom. The MatriXX PT has the same detector array arrangement as the standard MatriXX^Evolution but utilizes a smaller 2 mm plate spacing instead of 5mm. Because the bias voltage of the MatriXX PT and Evolution cannot be changed, PPC40 and FC65-G ionization chambers were used to assess recombination effects. The PPC40 is a parallel plate chamber with an electrode spacing of 2mm, while the FC65-G is a Farmer chamber FC65-G with an electrode spacing of 2.8 mm. Three bias voltages (500, 200, and 100 V) were used for both detectors to determine which radiation type (continuous, pulse or pulse-scanned beam) could closely estimate P_ion from the ratios of charges collected. In comparison with the MatriXX^Evolution, a significant improvement in measurement of absolute dose with the MatriXX PT was observed. While dose uncertainty of the MatriXX^Evolution can be up to 4%, it is < 1% for the MatriXX PT. Therefore the MatriXX^Evolution should not be used for QA of PBS for conditions in which ion recombination is not negligible. Farmer chambers should be used with caution for measuring the absolute dose of PBS beams, as the uncertainty of P_ion can be > 1%; chambers with an electrode spacing of 2 mm or smaller are recommended.