Program Information
Evaluating the Accuracy of Absolute Portal Dosimetry On the EDGE Linear Accelerator
M Bellon1*, P Keller2 , C Baltes2 , I J Chetty1 , J J Gordon1 , (1) Henry Ford Health System, Detroit, MI, (2) Laboratory GmbH, Baden-dattwil, Baden-Dattwil
Presentations
TH-CD-304-1 (Thursday, July 16, 2015) 10:00 AM - 12:00 PM Room: 304
Purpose: To validate the accuracy of the Varian (Palo Alto, CA) absolute portal dosimetry (APD) platform for the EDGE linear accelerator and evaluate the performance of an improved, non-transit EPID dosimetry system which is optimized to mitigate the known limitations of the current, clinically available solution.
Methods: Measurements were performed using the Varian EDGE in 6X FFF mode and aS1200 MV detector—a new, high energy, backscatter-shielded, amorphous silicon EPID (43x43 cm²). Integrated images were acquired for 124 clinical, IMRT/VMAT fields. The raw images were corrected for pixel sensitivity variation and off-axis differential energy response and converted to dose in water at 5cm depth using an empirical dose model. Delivered doses were compared to reference doses calculated in a water phantom using the TPS dose calculation algorithm. Per-beam analysis was performed using gamma evaluation with 3%/1mm dose difference/distance-to-agreement specifications. Additionally, composite EPID doses were calculated for 16 IMRT/VMAT treatment plans and benchmarked against gafchromic EBT3 film measurements, with agreement being analyzed using 3%/3mm, 2%/2mm and 3%/1mm gamma criteria.
Results: The converted EPID doses showed good agreement with calculated doses to water. An average of 98.1% (± 4.4%) of pixels passed per-beam gamma analysis with a stringent 3%/1mm criteria. The comparison of EPID measurements with film also yielded high pass rates for varying plan complexities and over a wide range of doses (maximum planar dose: 2.6-36.4 Gy) for each level of gamma criteria. Average gamma indices were 99.7%, 96.1% and 90.3% for 3%/3mm, 2%/2mm and 3%/1mm parameters, respectively.
Conclusion: The APD platform on the EDGE includes hardware upgrades, advanced detector calibration techniques and more sophisticated calculation capabilities which allow for accurate, high-resolution absolute dose verification for conventional as well as SRS and SBRT IMRT/VMAT cases. Further investigation is needed to evaluate the error detectability of this QA method.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by Varian Medical Systems (Palo Alto, CA)
Contact Email: