Determination of the electronic portal imaging device pixel‐sensitivity‐map for quality assurance applications. Part 1: Comparison of methods

Abstract

Calibration of a radiotherapy electronic portal imaging device (EPID) using the pixel-sensitivity-map (PSM) in place of the flood field correction improves the utility of the EPID for quality assurance applications.Multiple methods are available for determining the PSM and this study provides an evaluation to inform on which is superior. Methods: Three different empirical methods ("Calvary Mater Newcastle"[CMN], "Varian,"and "WashU") and a Monte Carlo-based method of PSM determination were investigated on a single Varian TrueBeam STx linear accelerator (linac) with an aS1200 EPID panel. PSM measurements were performed for each empirical method three successive times using the 6 MV beam. The resulting PSM from each method was compared to the Monte Carlo method as a reference using 2D percentage deviation maps and histograms plus crossplane profiles. The repeatability of generated PSMs was also assessed via 2D standard deviation (SD) maps and histograms. Additionally, the Beam-Response generated by removal of the PSM from a raw EPID image for each method was visually contrasted. Finally, the practicality of each method was assessed qualitatively and via the measured time required to acquire and export the required images. Results:The median pixel-by-pixel percentage deviation between each of the empirical PSM methods and the Monte Carlo PSM was -0.36%, 0.24%, and 0.74% for the CMN, Varian, and WashU methods, respectively. Ninety-five percent of pixels were found to be repeatable to within -0.21%, 0.08%, 0.19%, and 0.35% (1 SD) for the CMN, Monte Carlo, Varian, and WashU methods, respectively. The WashU method was found to be quickest for data acquisition and export and the CMN the slowest. Conclusion: For the first time four methods of generating the EPID PSM have been compared in detail and strengths and weaknesses of each method have been identified. All methods are considered likely to be clinically acceptable and with similar practical requirements.

Determination of the electronic portal imaging device pixel‐sensitivity‐map for quality assurance applications. Part 2: Photon beam dependence

“…Therefore, if there was no dependency on the dose rate for the beam response then the percent deviation histogram would be expected to be centered on zero to within the CMN method repeatability uncertainty presented in the Part 1 study. 12 The beam response deviations of Figure 11 are marginally outside repeatability (median deviation of 0.26% compared to 95% of pixels repeatable to within 0.21% as per the Part 1 study) hence indicating a small beam‐response dose rate dependence. This could be due to small differences in the beam at the 400 MU/min dose rate that have not been accurately characterized by the 2400 MU/min flood field that was removed from each image as part of the CMN method.…”

“…Monte Carlo results were not presented in Table 2 as they are inherently symmetric. This weakness of the Monte Carlo method has been discussed in detail in Part 1 of this study, 12 but the results of Table 1 which show measured asymmetry from all methods provide an example why an artificially symmetric beam‐response is problematic for a QA application.…”

“…However, this region is of least importance for QA applications as it is outside a 30 × 30 cm 2 field size. It is worth reminding from Part 1 of this study 12 that the Monte Carlo beam‐response is idealized, in that perfect beam symmetry and ideal beam energy is assumed. As this will not actually be the case for a real beam as used with the empirical PSM methods, then any real asymmetries or beam energy variation from ideal will result in variation in the plots of Figure 10 .…”

“…Details on the methodology of the four OSM methods investigated were presented in Part 1 of this study. 12 Each empirical method was applied for multiple MV photon beams: 6 MV and 10 MV and 6 and 10 MV FFF. In the case of the Monte Carlo method, the beam‐response simulations were repeated for each beam.…”

“…In Part 1 of this study, 12 the PSM results for the three empirical methods were compared against results from the Monte Carlo method for the 6 MV beam. In Part 2 of this study, this analysis has been extended to the other available photon beams.…”