Determination of Integral Depth Dose in Proton Pencil Beam Using Plane-parallel Ionization Chambers.

Purpose: This study aimed to determine the integral depth-dose curves and assess the geometric collection efficiency of different detector diameters in proton pencil beam scanning.

Materials and methods: The Varian ProBeam Compact spot scanning system was used for this study. The integral depth-dose curves with a proton energy range of 130 to 220 MeV were acquired with 2 types of Bragg peak chambers: 34070 with 8-cm diameter and 34089 with 15-cm diameter (PTW), multi-layer ionization chamber with 12-cm diameter (Giraffe, IBA Dosimetry), and PeakFinder with 8-cm diameter (PTW). To assess geometric collection efficiency, the integral depth-dose curves of 8- and 12-cm chamber diameters were compared to a 15-cm chamber diameter as the largest detector.

Results: 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. Moreover, PTW Bragg peak chamber type 34089 had increased geometric collection efficiency up to 3.8%, 6.1%, and 3.1% when compared to PTW Bragg peak chamber type 34070, PTW PeakFinder, and IBA Giraffe, respectively.

Conclusion: A larger plane-parallel ionization chamber could increase the geometric collection efficiency of the detector, especially at intermediate depths and high-energy proton beams.