Abstract ID: 230 Experimental and Monte Carlo investigation of the depth-dependent fluence perturbation of parallel-plate chambers in clinical electron beams

von Voigts-Rhetz Philip; Vorwerk Hilke; Zink Klemens

doi:10.1016/j.ejmp.2017.11.031

Abstract ID: 230 Experimental and Monte Carlo investigation of the depth-dependent fluence perturbation of parallel-plate chambers in clinical electron beams

Phys Med. 2018 Jan:45 Suppl 1:S4. doi: 10.1016/j.ejmp.2017.11.031.

Authors

von Voigts-Rhetz Philip¹, Vorwerk Hilke², Zink Klemens³

Affiliations

¹ Technische Hochschule Mittelhessen - University of Applied Sciences, Giessen, Germany; University Medical Center Marburg Philipps-University, Marburg, Germany.
² University Medical Center Marburg Philipps-University, Marburg, Germany.
³ Technische Hochschule Mittelhessen - University of Applied Sciences, Giessen, Germany.

PMID: 29413856
DOI: 10.1016/j.ejmp.2017.11.031

Abstract

The electron fluence inside a parallel-plate ionization chamber positioned in a water phantom and exposed to a clinical electron beam deviates from the unperturbed fluence in water in absence of the chamber. One reason for the fluence perturbation is the "in-scattering effect" as described in ICRU 35. Aim of this work is a detailed experimental and with Monte Carlo simulations investigation of this effect. The experiments and Monte Carlo Simulations, with EGSnrc [1], were performed in a solid-water phantom (RW3) sized 30 × 30 × 30 cm³. One of the RW3 plates of height h = 0.5 cm had a hole with radius r = 1 cm which acts as the air cavity in the phantom. The phantom was placed below an electron applicator of field size 10 × 10 cm² and irradiated with 6 MeV electrons from an Elekta Synergy accelerator. The air cavity was positioned at depths of 1.0, 1.5, 2.0 and 2.5 cm in the phantom. For the exerpiments the spatial resolved dose directly below the cavity was scored with GAFCHROMIC ETB3 films. The films were scanned with an Epson 10000xl-scanner (resolution 600 dpi, color depth 48 bit); a dose response calibration curve for the red channel was applied. At 1.0 cm depth the relative dose profile shows the well known dose increase at the air/RW3 boundary of about 4% relative to the dose at the center of the cavity and a de-crease of about 8% outside the air cavity within the RW3-phantom. The amplitude of this "dose oscillation" decreases with increasing depth of the cavity within the phantom and disappears at about 2 cm, which is less than the half value thickness of the electron beam. The results are in agreement with already published Monte Carlo results [2]. The results are partly in contradiction to the ideas summarized in ICRU 35 but confirm recent Monte Carlo simulations from Zink et al.