Long-term performance monitoring of a-Si 1200 electronic portal imaging device for dosimetric applications

Abstract

Purpose: Recently, dosimetri applications of the electronic portal imaging device (EPID) in radiotherapy have gained popularity. Confidence in the robust and reliable dosimetric performance of EPID detectors is essential for their clinical use. This study aimed to evaluate the dosimetric performance of the a-Si 1200 EPID and assess the long-term stability of its response.

Methods: Weekly measurements were performed on two clinically used TrueBeam linear accelerators (linacs) equipped with a-Si 1200 EPID detectors over a 2-year period. They included dark and flood calibration fields, and EPID response to an open field corrected for the long-term machine output drift measured with the secondary absolute dosimeters: an ion chamber and an ion chamber array. All measurements were performed using five photon beam energies and two imaging modes: continuous and dosimetry. The measurements were analyzed for constancy and the presence of long-term trends. Comparisons were made between the two linacs for each beam energy. Pixel sensitivity matrices (PSM) were determined semi-annually and analyzed for long-term constancy for both treatment machines.

Results: The long-term variation of the dark and flood field signals, integrated across the EPID plane, over the entire observation period did not exceed 0.17% and 0.79%, respectively. The output-corrected EPID response showed long-term variation from 0.28% to 0.36%, depending on beam energy, while the short-term variation was 0.04%-0.07% for EPID and 0.02%-0.06% for secondary dosimeters. The long-term variation of secondary dosimeters was 0.2%-0.3%. PSMs were found to be stable to within 1% for 97.8% of pixels and 2% for 100% of pixels.

Conclusion: Techniques to monitor and assess the long-term performance of the a-Si 1200 EPID as a dosimeter were developed and implemented using two TrueBeam linacs. The long-term variation of the EPID response was within clinical tolerance indicated in AAPM TG-142 report, and the detector was shown to be stable and reproducible for routine clinical dosimetry.

Keywords: EPID; EPID dosimetry; EPID response stability; Radiotherapy; linac QA; linac output; machine QA.

FIGURE 1

Integral dark field measurements for all beam energies, dosimetry mode, Unit A. Note that data for the FFF beams overlay each other on the lower line while data for the non‐FFF beams overlay each other on the higher line.

FIGURE 2

(a) Pixelwise standard deviation for dosimetry 10FFF dark field on Unit A, (b) CAX crossplane and inplane profiles of data in (a).

FIGURE 3

Dosimetry flood field measurements for flattened beams 6X, 10X, and 23X, Unit A.

FIGURE 4

Color plot of pixelwise dosimetry flood field standard deviations, Unit J: (a) 10X beam and (b) 10FFF beam.

FIGURE 5

Distributions of standard deviations of individual EPID pixel flood field measurements in dosimetry mode for all beam energies, for Unit A (a) and Unit J (b).

FIGURE 6

Continuous flood field measurements for flattened beams 6X, 10X, and 23X, Unit J.

FIGURE 7

Distributions of standard deviations of individual EPID pixel flood field measurements in continuous mode for all beam energies, for Unit A (a) and Unit J (b).

FIGURE 8

EPID measured 6X beam output, Unit A, uncorrected and corrected for the machine output using (a) ion chamber and (b) MatriXX measurements.

FIGURE 9

EPID measured 6X beam output, Unit J, uncorrected and corrected for the machine output using (a) ion chamber and (b) MatriXX measurements.

FIGURE 10

Colorplot of greatest pixelwise changes observed in PSM for (a) Unit A and (b) Unit J, and Distribution of greatest pixelwise changes observed in PSM for (c) Unit A and (d) Unit J.