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SU‐E‐T‐208: Optimizing Parameters in Imaging for Accurate EPID Exit Dosimetry Application
Author(s) -
Li K,
Beardmore A,
Able A
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612158
Subject(s) - dosimetry , calibration , nuclear medicine , image guided radiation therapy , pixel , medicine , linear particle accelerator , medical imaging , optics , physics , mathematics , beam (structure) , radiology , statistics
Purpose: Besides being used for patient treatment alignment, EPIDs treatment quality control is advancing to application of exit dosimetry analysis. The absolute dosimetry is determined by the accuracy of the calibration between portal image pixel value and dose. In this study, a generic IMRT field was selected as an evaluation reference, the dosimetric effect of pixel value and dose calibration was investigated by varying energy, dose rate and total delivered dose. Method and Materials:Varian EPID and Imaging acquisition system (IAS3‐aS1000) were used to create images from dose delivered from a Varian Trilogy Linac machine. The doses were delivered with a 60 degree enhance dynamic wedges (EDW) field varied with different energies, total doses and dose rates. And the images were exported to the RIT system (RIT113V5.3) for analysis. The dosimetric calibration curves were used to analyze the effect of each parameter. The slopes between the calibration curves were also calculated. then a prostate IMRT field was employed for absolute dosimetry verification. Results: At dose rate of 400MU per minute, the magnitudes of slope range from 10 pixel value per cGy to 27 pixel value per cGy for calibration from 50MU to 5000MU. When dose rates vary from 100 to 600 MU per minute, the slopes change from 29 pixel value per cGy to 1 pixel value per cGy. A selected point dose from an IMRT field was compared with EPID measurement and was within 5% using calibrations at 200MU delivered at 600 MU per minute, or 100MU delivered at 400MU per minute. Conclusions: Accurate dosimetric application for EPID requires meticulous selection of imaging parameters. For high precision exit dosimetry based on EPID imaging, further investigation is needed for total Monitor Unit and delivery temporal effect from IMRT Fields.