Evaluation of a fast method of EPID‐based dosimetry for intensity‐modulated radiation therapy

Benjamin E Nelms,Wolfgang A Tomé,Karl H Rasmussen

doi:10.1120/jacmp.v11i2.3185

Abstract

Electronic portal imaging devices (EPIDs) could potentially be useful for intensity‐modulated radiation therapy (IMRT) QA. The data density, high resolution, large active area, and efficiency of the MV EPID make it an attractive option. However, EPIDs were designed as imaging devices, not dosimeters, and as a result they do not inherently measure dose in tissue equivalent media. EPIDose (Sun Nuclear, Melbourne, FL) is a tool designed for the use of EPIDs in IMRT QA that uses raw MV EPID images (no additional build‐up and independent of gantry angle, but with dark and flood field corrections applied) to estimate absolute dose planes normal to the beam axis in a homogeneous media (i.e. similar to conventional IMRT QA methods). However, because of the inherent challenges of the EPID‐based dosimetry, validating and commissioning such a system must be done very carefully, by exploring the range of use cases and using well‐proven “standards” for comparison. In this work, a multi‐institutional study was performed to verify accurate EPID image to dose plane conversion over a variety of conditions. Converted EPID images were compared to 2D diode array absolute dose measurements for 188 fields from 28 clinical IMRT treatment plans. These plans were generated using a number of commercially available treatment planning systems (TPS) covering various treatment sites including prostate, head and neck, brain, and lung. The data included three beam energies (6, 10, and 15 MV) and both step‐and‐shoot and dynamic MLC fields. Out of 26,207 points of comparison over 188 fields analyzed, the average overall field pass rate was 99.7% when 3 mm/3% DTA criteria were used (range 94.0–100 per field). The pass rates for more stringent criteria were 97.8% for 2 mm/2% DTA (range 82.0–100 per field), and 84.6% for 1 mm/1% DTA (range 54.7–100 per field). Individual patient‐specific sites as well, as different beam energies, followed similar trends to the overall pass rates.PACS number: 87.53.Dq; 87.66.Jj

Highlights

141 Nelms et al.: Evaluation of Electronic portal imaging devices (EPIDs)-based intensitymodulated radiation therapy (IMRT) QA methodsystems and treatment planning systems (TPS)
1 mm/1% does not seem to be a clinically used criteria when doing planar dose comparisons.[17]. To summarize, our findings suggest that one can expect similar pass rates for different treatment sites in terms of the performance of the EPID IMRT QA system when compared to those measured with an IMRT QA 2D diode array
Our results demonstrate that the EPID dosimetry methodology described and analyzed in this work can be used to efficiently carry out the absolute dosimetry for IMRT treatment field quality assurance

Summary

Introduction

141 Nelms et al.: Evaluation of EPID-based IMRT QA methodsystems and treatment planning systems (TPS). One general IMRT QA method that is essential to commissioning and common in per-plan QA (for nonrotational plans) is to treat IMRT fields one by one onto a measurement phantom, measuring a dose plane normal to the beam direction and comparing to a TPS-calculated dose plane. Such a method verifies dose across the modulated 2D profile of each beam and enables the clinical physicist to: 1) assess the ability of the TPS to calculate dose given complex beam modulation, 2) assess that the treatment delivery system is capable of delivering the complex fields, and 3) diagnose and troubleshoot failures of the TPS and/or treatment delivery system. The use of small detectors in measurements is essential to avoid the problem of volume averaging that is present when using large detectors

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