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Experimental Investigation of the Depth-Dependent Fluence Perturbation of Parallel-Plate Chambers in Clinical Electron Beams

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P von Voigts-Rhetz

P von Voigts-Rhetz1,2*, H Vorwerk2 , K Zink1,2 , (1) Technische Hochschule Mittelhessen - University of Applied Sciences, Giessen, Hessen, (2) University Medical Center Marburg Philipps-University, Marburg, Hessen,


SU-I-GPD-T-59 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: 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 “inscattering effect” as described in ICRU 35. Aim of this work is a detailed experimental investigation of this effect.

Methods: The experiments were performed in a solid-water phantom (RW3) sized 30x30x30cm³. 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 spatial resolved dose directly below the cavity was scored with GAFCHROMIC ETB3 films. The phantom was placed below an electron applicator of field size 10 x 10 cm2 and irradiated with 6 MeV electrons from an Elekta Synergy accelerator. For the measurements the air cavity was positioned at depths of 1.0, 1.5, 2.0 and 2.5 cm in the phantom. The films were scanned with an Epson 10000xl-scanner (resolution 600dpi, color depth 48bit); a dose response calibration curve for the red channel was applied.

Results: 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 decrease 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 experimental results are in agreement with already published Monte Carlo results [Zink Med.Phys. 2014].

Conclusion: The experimental results are partly in contradiction to the ideas summarized in ICRU 35 but confirm recent Monte Carlo simulations from Zink et al.

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