Evaluation of integral depth-dose curves in proton pencil beam scanning using plane-parallel ionization chambers

Abstract

Proton therapy is an advanced form of radiation therapy treatment that can potentially decrease side effects and increase tumor control probability. The integral depth-dose curve is an essential parameter that has to be determined and introduced into the proton treatment planning system for the dose calculation. Besides the halo, the integral depth-dose curve measurements should be performed with a large-diameter plane-parallel ionization chamber. The purpose of this research is to determine the integral depth-dose curves and assess the geometrical collection efficiency at intermediate depths of different detector diameters in proton pencil beam scanning. The integral depth-dose curves with a proton energy range of 70 to 220 MeV were measured using Bragg peak chambers type 34070 with 8 cm diameter and 34089 with 15 cm diameter (PTW, Germany), multi-layer ionization chamber with 12 cm diameter (Giraffe, IBA dosimetry), and PeakFinder with 8 cm diameter (PTW, Germany). To assess the geometrical collection efficiency, the ratios of a depth-dose curve from two different chambers were investigated. The results found that at intermediate depths of 130, 150, 190, and 220 MeV, PTW Bragg peak chamber type 34089 provided the highest integral depth-dose curves followed by IBA Giraffe, PTW Bragg peak chamber type 34070, and PTW PeakFinder. Besides that PTW Bragg peak chamber type 34089 had increased geometrical collection efficiency up to 3.8%, 6.1%, and 3.1% compared to PTW Bragg peak chamber type 34070, PTW PeakFinder, and IBA Giraffe, respectively. In conclusion, a larger plane-parallel ionization chamber could increase the geometrical collection efficiency of the detector, especially at intermediate depths and high-energy proton beams.