Compensator quality control with an amorphous silicon EPID

Abstract

The calibration and quality control of compensators is conventionally performed with an ion chamber in a water-equivalent phantom. In our center, the compensator factor and four off-axis fluence ratios are measured to verify the central axis beam modulation and orientation of the compensator. Here we report the investigation of an alternative technique for compensator quality control using an amorphous silicon electronic portal imaging device (a-Si EPID). Preliminary experiments were performed to identify appropriate EPID operating parameters for this relative dosimetric study and also to quantify EPID operation. The pixel value versus energy fluence response of the EPID for both open and compensated fields was then determined, and expressed via calibration curves. For open fields the response was seen to be linear, whereas for compensated fields it exhibited a small quadratic component. To account for field size effects, we measured EPID scatter factors. These exhibited small but non-negligible dependencies on compensator thickness and source-detector distance. Finally, a number of test and clinical compensators were evaluated to assess the suitability of the EPID for compensator quality control. Our results indicate that the a-Si EPID can measure clinical compensator factors and off-axis energy fluence ratios to within 2% of values measured by a Farmer chamber on average, and so is a suitable ion chamber replacement.

Off‐axis dose response characteristics of an amorphous silicon electronic portal imaging device

“…The linearity of the Varian aS500 EPID with increasing MU setting at central axis has been examined by a number of authors. [3][4][5] The linearity of a newer dosimetric mode that does not exhibit frame loss and acquires extra frames following beam-off to ensure that the final frame is completely read out was examined by van Esch et al 6 They found that the detector had a linear curve for doses from 2 to 300 MU; however, differences below 30 MU were found due to signal rounding. The linearity of the Elekta iView EPID has been examined by McDermott et al 7 and more recently Winkler et al 8 They reported that the reduction in EPID signal for low doses ͑Ͻ15 MU͒ was due to a dose rate or dose per frame response.…”

Monte Carlo modelling of a‐Si EPID response: The effect of spectral variations with field size and position

“…1 As the image is instantly obtained, there is no need to process a film and therefore it is possible to verify the accuracy of positioning during the course of the field delivery. As the technology develops, EPIDs are not only used for patient positioning verification but also for a variety of other applications: linac quality assurance ͑QA͒, [2][3][4] compensator design and verification, 5,6 and patient dosimetry. [7][8][9][10][11][12][13][14][15][16][17][18] This is of special interest for intensity modulated radiation treatments ͑IMRT͒.…”

On Radiotherapy Dose Verification With a Flat-Panel Imager

“…31,41 For the a-Si EPIDs of another vendor, a variation in response of 0.8% (1 SD) over a period of 1 month has been reported16, and a drift depending on the beam quality up to 4% over a period of 5 months. 82 …”

“…42,43,73,82,122,131 All of these studies used the Varian EPID, which has a different scintillator from the Elekta and Siemens detectors. The EPID signal for these studies was measured over different dose ranges, energies and dose-rate settings compared to measurements with the Elekta EPIDs.…”