Program Information
Efficient EPID-Based QA for SBRT and SRS Treatments
T Ostwald1,2*, R Artschan1 , R Jones1 , P Greer1,2 , (1) Calvary Mater Newcastle, Newcastle, Australia (2) Newcastle University, Newcastle, Australia
Presentations
MO-RPM-GePD-T-4 (Monday, July 31, 2017) 3:45 PM - 4:15 PM Room: Therapy ePoster Lounge
Purpose: With stereotactic treatments becoming more routine in radiotherapy departments, a method for streamlining the labour-intensive patient specific QA process is vital. This work reports on the implementation of an efficient EPID-based QA method for VMAT SBRT and SRS treatment plans.
Methods: SBRT and SRS VMAT treatments are delivered in-air to the EPID and images acquired using a frame-grabber PC. In-house MATLAB code converts the images into a 3D dose distribution in a cylindrical virtual water phantom. The code calculates an absolute dose at a point of interest and 2D Gamma on planes through the point as well as 3D Gamma. A series of VMAT clinical plans were analysed with the code and compared with EBT3 Gafchromic film and CC04 ion chamber measurements in a Lucy Phantom, and ArcCHECK. The plans included SBRT lung, spine and prostate plans, and both dynamic conformal arc and VMAT SRS plans.
Results: For determining positional accuracy the EPID method was found to be consistent with film ( ±1.5mm). EPID point doses for dynamic conformal arc plans for PTVs 0.8cm to 2cm in diameter were similar to CC04 measurements when compared with planned doses (1.2% ±0.5%SD EPID compared to -0.1% ±0.9%SD CC04). Gamma analysis of SBRT plans showed 90% of plans had gamma pass rates (3%, 2mm) better than 95% and all were better than 92%. The code was able to identify failures (measured doses 6-9% higher than planned) in VMAT SRS cases, however, since the code computes all fields as coplanar, the clinical significance of gamma failures in non-coplanar plans can be difficult to determine.
Conclusion: EPID-based QA was found to be an efficient method of patient specific QA for coplanar SBRT plans and simple SRS plans, but more work is needed for complex non-coplanar SRS plans.
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