Simulations of Real-Time Geometric and Dosimetic Verification System Using EPID
T Fuangrod1*, H Woodruff1, E VanUytven2, B McCurdy2, D O'Connor1, P Greer3, (1) The university of Newcastle, Australia,(2) CancerCare Manitoba, Winnipeg, MB, Canada,(3) Newcastle Mater Hospital, Newcastle, Australia,MO-G-213AB-3 Monday 5:15:00 PM - 6:00:00 PM Room: 213AB
Purpose: To demonstrate a new method for real-time geometric and dosimetric verification of IMRT and VMAT using synchronization between predicted and measured EPID images.
Methods: Predicted EPID images were calculated using a comprehensive physics-based model.Each predicted image represents the integrated signal expected from the delivery between control points.The measured images are acquired in cine mode and compared to the set of predicted images in real-time.The system performs geometric verification prior to dosimetric verification.When the measured image is acquired,the algorithm automatically detects the MLC leaf positions.A comparison between the leaf positions of the measured image and control points in the MLC file is made using the cosine similarity technique.The similarity index(SI) provides geometric MLC verification and synchronization between the measured and predicted images,as a uniform dose-rate cannot be assumed for IMRT or VMAT deliveries.The SI threshold was based on a series of experiments including 21 dynamic-IMRT fields defining pass/fail boundary(5 brain, 8 H&N, and 8 prostate cases).If geometric verification is successful, dosimetic verification is performed with the Gamma comparison(3%,3mm).The system reports the verification result in real-time.
Results: The system was simulated by MATLAB/SIMULINK and detected geometric and dosimetric errors during delivery.Both artificially introduced errors and clinical data were used for testing and analysis of the system performance.For a tested prostate field,the cumulative dose comparisons showed the minimum and maximum number of points with Gamma index<1 as 93.5% and 98.5%,respectively.For individual dose comparisons on the same field,the values were 87% and 97%,respectively.
Conclusion: This method includes automatic MLC leaf positioning, synchronization,and dosimetric verification.The pass/fail boundary of geometry was calculated based on the experiments.This system is a useful approach to detect unexpected possible errors occuring in the clinical setting and to prevent patient overdoses during radiotherapy especially in complex deliveries such as arc-IMRT.