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The Electronic Portal Imaging Device (EPID), primarily used for patient setup during radiotherapy sessions is also used for dosimetric measurements. In the present study, the feasibility of EPID in both machine and patient-specific quality assurance (QA) are investigated. We have developed a comprehensive software tool for effective utilisation of EPID in our institutional QA protocol.
Materials and methods
Portal Vision aS1000, amorphous silicon portal detector attached to Clinac iX—Linear Accelerator (LINAC) was used to measure daily profile and output constancy, various Multi-Leaf Collimator (MLC) checks and patient plan verification. Different QA plans were generated with the help of Eclipse Treatment Planning System (TPS) and MLC shaper software. The indigenously developed MATLAB programs were used for image analysis. Flatness, symmetry, output constancy, Field Width at Half Maximum (FWHM) and fluence comparison were studied from images obtained from TPS and EPID dosimetry.
The 3 years institutional data of profile constancy and patient-specific QA measured using EPID were found within the acceptable limits. The daily output of photon beam correlated with the output obtained through solid phantom measurements. The Pearson correlation coefficients are 0.941 (p = 0.0001), 0.888 (p = 0.0188) and 0.917 (p = 0.0007) for the years of 2014, 2015 and 2016, respectively. The accuracy of MLC for shaping complex treatment fields was studied in terms of FWHM at different portions of various fields, showed good agreement between TPS-generated and EPID-measured MLC positions. The comparison of selected patient plans in EPID with an independent 2D array detector system showed statistically significant correlation between these two systems. Percentage difference between TPS computed and EPID measured fluence maps calculated for number of patients using MATLAB code also exhibited the validity of those plans for treatment.
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