Program Information
Commissioning and Evaluation of EPID-Based in Vivo Dosimetry Software Using a Tissue-Maximum Ratio Approach
M Held1*, J Cheung1 , F Husson2 , O Morin1 , (1) University of California San Francisco, San Francisco, CA, (2) Dosisoft SA, Cachan, France
Presentations
SU-G-BRB-6 (Sunday, July 31, 2016) 4:00 PM - 6:00 PM Room: Ballroom B
Purpose: To commission and evaluate an in vivo EPID-based transit dosimetry software (EPIgray, DOSIsoft, Cachan, France) using simple fields and TG119-based IMRT treatment plans.
Methods: EPIgray was commissioned on a Truebeam based on finite tissue-maximum ratio (fTMR) measurements with solid water blocks of thicknesses between 0 and 37 cm. Field sizes varied from 2x2 to 20x20 cm². Subsequently, treatment plans of single and opposed beams with field sizes between 4x4 and 15x15 cm² as well as IMRT plans were measured to evaluate the dose reconstruction accuracy. Single field dose predictions were made for anterior-posterior and lateral beams. IMRT plans were created based on TG119 recommendations. The reconstructed dose was compared to the planned dose for selected points at isocenter and away from isocenter.
Results: For single square fields, the dose in EPIgray was reconstructed within 3% accuracy at isocenter relative to the planned dose. Similarly, the relative deviation of the total dose was accurately reconstructed within 3% for all IMRT plans with points placed inside a high dose region near the isocenter. Predictions became less accurate than 5% when the evaluation point was outside the majority of IMRT beam segments. Additionally, points 5 cm or more away from the isocenter or within an avoidance structure were predicted less reliably.
Conclusion: EPIgray formalism accuracy is adequate for an efficient error detection system. It provides immediate intra-fractional feedback on the delivery of treatment plans without affecting the treatment beam. Besides the EPID, no additional hardware is required, which makes it accessible to all clinics. The software evaluates point dose measurements to verify treatment plan delivery and patient positioning within 5% accuracy, depending on the placement of evaluation points. EPIgray is not intended to replace patient-specific quality assurance but should be utilized as an additional layer of safety for continuous patient treatment verification.
Funding Support, Disclosures, and Conflict of Interest: This research is supported by DOSIsoft.
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