TH‐CD‐304‐01: Evaluating the Accuracy of Absolute Portal Dosimetry On the EDGE Linear Accelerator.
Bellon M, Keller P, Baltes C, Chetty IJ, and Gordon JJ. Th-cd-304-01: Evaluating the accuracy of absolute portal dosimetry on the edge linear accelerator. Med Phys 2015; 42(6):3722.
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.
Measurements were performed using the Varian EDGE in 6X FFF mode and aS1200 MV detector—a new, high energy, backscatter‐shielded, amorphous silicon EPID (43×43 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.
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.
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.
This work was supported in part by Varian Medical Systems (Palo Alto, CA)