Quality assurance of electron and photon beam energy using the BQ‐Check phantom

Richard J Speight,Ashraf Esmail,Steve J Weston

doi:10.1120/jacmp.v12i2.3366

Richard J Speight, Ashraf Esmail + Show 1 more

Open Access

https://doi.org/10.1120/jacmp.v12i2.3366

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Abstract

The BQ‐CHECK phantom (PTW Freiburg, Germany) has been designed to be used with a 2D ion chamber array to facilitate the quality assurance (QA) of electron and photon beam qualities (BQ). The BQ‐CHECK phantom has three wedges covering the diagonal axes of the beam: two opposed aluminum wedges used to measure electron energy and a single copper wedge used to measure photon energy. The purpose of this work was to assess the suitability of the BQ‐CHECK phantom for use in a routine QA program.A range of percentage depth dose (PDD) curves for two photon beams and four electron beams were measured using a MP3 plotting tank (PTW Freiburg). These beams were used to irradiate a STARCHECK array (PTW Freiburg) with and without the BQ‐CHECK phantom on top of the array. For photons, the ratio of the signals from two chambers underneath the copper wedge was used as an effective TPR measurement (TPReff) and, for electrons, the full width at half maximum of the profile (EFWHM) underneath the aluminum wedges was used as an electron energy constancy measurement. PDD measurements were compared with TPReff and EFWHM to assess the sensitivity of the BQ‐CHECK phantom.The clinical tolerances of TPReff were determined for 6 MV (0.634–0.649), and 10MV (0.683–0.692). For electrons, the clinical tolerances of EFWHM were determined for 6 MeV (94.8–103.4 mm), 8 MeV (105.5–114.0 mm), 10 MeV (125.4–133.9 mm) and 12 MeV (138.8–147.3 mm).Electron and photon energy metrics are presented which demonstrate that the BQ‐CHECK phantom could be used to form part of an efficient routine monthly QA program. Acceptable beam quality limits for various nominal beam energies were established and at these limits, modified profiles were acquired using the STARCHECK array. From the modified profiles, EFWHM and TPReff were determined for the electron and photon beams, respectively. It was demonstrated that both EFWHM and the TPReff have a linear relationship with conventional beam quality metrics.PACS numbers: 87.56.bd, 87.56.‐v

Highlights

240 Speight et al.: quality assurance (QA) using the BQ-Check phantom energy characterization is the measurement of depth ionization curves or depth dose curves in a water phantom using a suitable detector,(4) and usually a measurement such as dose at depth is taken as the metric
It has been shown that the BQ-CHECK phantom combined with the STARCHECK array can be used in the quality assurance of photon and electron beam energy
EFWHM and TPReff, have beam evaluated against electron and photon beam energy, respectively

Summary

Introduction

240 Speight et al.: QA using the BQ-Check phantom energy characterization is the measurement of depth ionization curves or depth dose curves in a water phantom using a suitable detector,(4) and usually a measurement such as dose at depth is taken as the metric. Measurements taken from the profile underneath the wedge can be correlated with the beam energy.[5,6,7] Wells et al[8] introduced a double wedge technique for measuring electron beam energy that is invariant to phantom alignment in the wedge direction, and had a similar sensitivity to water tank measurements for electron energies between 6 and 20 MeV. They used a phantom with two PMMA wedges attached to a diode array to produce profiles at a fixed source-to-surface distance (SSD) and field size. The half width at half maximum (which they called EC50) was measured and shown to be proportional to R50 (the depth of 50% dose along the central axis) for the electron energies tested.

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