Geometrical Efficiency of Plane-Parallel Ionization Chambers in Proton Scanning Beam

Abstract

For commissioning of a proton therapy unit depth dose distributions must be determined and introduced into the Treatment Planning System. In pencil beam scanning (PBS) technique, integral depth dose (IDD) acquisition should be performed with detector large enough to ensure entire beam laterally broadened by scattered and secondary contributions. The purpose of this article is to quantify, using measurements and Monte Carlo transport calculations, the ionization chamber's (IC) geometrical efficiency versus the chamber radius and proton beam energy. The geometrical efficiency of 0.99 was determined for energies up to 160 and 190 MeV for 4.08 and 6 cm radius IC. Much lower geometrical efficiency was obtained for the energy of 226.08 MeV and results in charge loss of 5.8 and 3.6%, respectively. Relative IDD differences between IC 4.08 and 6 cm in radius increase with proton energy and reach 2.4% at the mid-range depth for 226.08 MeV.

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

“…Therefore, the increasing detector diameter improves geometric collection efficiency depending on beam energy and depth, and the most significant difference was found at intermediate depths of the highest proton energy. It means that the PTW Bragg peak chamber type 34089 with the largest diameter could collect more secondary protons and reduce the inaccuracy of the halo effect, which is mainly produced by nuclear and coulomb interactions with the detection medium (eg, water); these findings agree with those of Baumer et al [4], Langner et al [2], and Mojzeszek et al [13]. Therefore, it can increase the accuracy of the proton dose model and dose calculation in the clinic.…”

“…The results obtained agreed with those of Baumer et al [4] who reported a collection efficiency up to 2.0% and 3.5% between PTW Bragg peak chamber type 34070 and IBA Stingray chamber for 180 and 226.7 MeV, respectively. In addition, Mojzeszek et al [13] achieved a higher collection efficiency of 5.8% between chamber diameters of 8 and 40 cm for 226.08 MeV by using data assessed with Monte Carlo calculations. The chamber diameters that these authors used to compare with the 8-cm chamber diameter could explain these differences.…”

“…All integral depth-dose curves were scaled to a maximum of 100% of the dose and shifted in depth to match R 80 from NIST to eliminate the range uncertainty before comparing the integral depth-dose curves measured with PTW Bragg peak chamber type 34070, PTW Bragg peak chamber type 34089, PTW PeakFinder, and IBA Giraffe [4,13].…”

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

“…Therefore, the increasing chamber diameter has improved geometrical collection efficiency depending on beam energy and depth. It means that the PTW Bragg peak chamber type 34089 with the largest diameter could collect more secondary proton from the halo, which is mainly produced by nuclear and coulomb interactions with the detection medium, and these agreed with Baumer C et al (4), Langner UW et al (2), and Mojzeszek N et al (13). The ratios between two chamber types in the peak region were close to one, indicating less halo effect, whereas higher collection efficiencies were observed at the distal fall-off region due to the limitation of detectors at the high dose gradient and step size of 5 mm.…”