Poster — Thur Eve — 03: A Monte Carlo Investigation of the Effects of a Novel In Vivo Transmission Detector on a 6 MV Photon Beam

Abstract

The effect of a transmission detector (TRD), a novel IMRT quality assurance tool to be used for in vivo dose measurements, was investigated. As a scattering material, the device will be a source of contaminant electrons and could potentially affect prescribed dose to patients during treatment. The goals of this investigation are to characterize the effect of the TRD on the clinical photon beam including IMRT fields, using Monte Carlo simulation. The linear accelerator head and the TRD were modeled using BEAMnrc. Particles scored at 70 cm and 100cm SSD for different field sizes (i.e. 5×5 cm2, 10×10 cm2, and 20 × 20 cm2) with and without the TRD were separated according to where they were created in the linac head. In addition, two IMRT fields with and without the TRD were simulated and their respective absolute dose distributions were compared. Without the TRD, air is the major source of contaminant electrons. When TRD is used, it absorbs nearly all the incident contaminant electrons, while becoming the major source of contaminant electrons. For both IMRT fields, the percentage dose differences of the calculated absolute doses with and without TRD at the isocenter were found to be within uncertainty of the measured transmission factor for the TRD. Although a major source of contaminant electrons, the TRD does not introduce excessive electron contamination into a clinical 6 MV photon beam and is therefore suitable for in vivo dose measurements if fully commissioned as an IMRT quality assurance tool. Research partly sponsored by IBA Dosimetry.