Assessment of dosimetrical performance in 11 Varian a-Si500 electronic portal imaging devices

Abstract

Dosimetrical characteristics of 11 Varian a-Si-500 electronic portal imaging devices (EPIDs) in clinical use for periods ranging between 10 and 86 months were investigated for consistency of performance and portal dosimetry implications. Properties studied include short-term reproducibility, signal linearity with monitor units, response to reference beam, signal uniformity across the detector panel, signal dependence on field size, dose-rate influence, memory effects and image profiles as a function of monitor units. The EPID measurements were also compared with those of the ionization chambers' to ensure stability of the linear accelerators. Depending on their clinical installation date, the EPIDs were interfaced with one of the two different acquisition control software packages, IAS2/IDU-II or IAS3/IDU-20. Both the EPID age and image acquisition system influenced the dosimetric characteristics with the newer version (IAS3 with IDU-20) giving better data reproducibility and linearity fit than the older version (IAS2 with IDU-II). The relative signal response (uniformity) after 50 MU was better than 95% of the central value and independent of detector. Sensitivity for all EPIDs reduced continuously with increasing dose rates for the newer image acquisition software. In the dose-rate range 100-600 MU min(-1), the maximum variation in sensitivity ranged between 1 and 1.8% for different EPIDs. For memory effects, the increase in the measured signal at the centre of the irradiated field for successive images was within 1.8% and 1.0% for the older and newer acquisition systems, respectively. Image profiles acquired at a lower MU in the radial plane (gun-target) had gradients in measured pixel values of up to 25% for the older system. Detectors with software/hardware versions IAS3/IDU-20 have a high degree of accuracy and are more suitable for routine quantitative IMRT dosimetrical verification.

Simple Proposal for Dosimetry with an Elekta iViewGTTM Electronic Portal Imaging Device (EPID) Using Commercial Software Modules

“…More recently, amorphous silicon (a-Si)-based EPIDs have been developed with excellent image quality [17] and requirements for dosimetric purposes [3,7,8,10,12,13,15]. So far only a few reports have addressed dosimetric applications for this type of detector from Elekta [14,18,24,25].…”

Dose-guided radiotherapy for lung tumors

“…Moreover, all the linacs were equipped with aSibased EPIDs (Varian Medical System, CA, USA). These systems supply good signal reproducibility and low saturation effects, as described in detail elsewhere [4,10,18,23].…”

Implementation and evaluation of a transit dosimetry system for treatment verification

“…Previous approaches have considered CCD camera based systems and liquid filled ionisation chamber matrixes. a-Si has proven popular due to its superior image quality and dose characteristics (linearity, uniformity, dose-rate dependence, field size dependence, relative dosimetry) [12] [13] [14].…”