Author: Ryan Burri, Nazanin Hoshyar, Jerry W. McCoy, Yulin Song, Huma Syed 👨🔬
Affiliation: Bay Pines VA Medical Center 🌍
Purpose: The study aimed to streamline traditionally manual, iterative, and time-consuming VMAT commissioning process. The goal was to identify an optimal set of MLC dosimetric parameters that minimizes discrepancies between calculated and measured dose distributions across diverse clinical scenarios. The study utilized a-Si 1200 EPID portal dosimetry for VMAT commissioning for a new Linac.
Methods:
Twenty-five previously treated clinical cases were selected, covering various treatment sites (abdomen, breast, brain, head/neck, lung, prostate, and spine).
The commissioning process involved multiple iterations of plan optimization, dose calculation, portal dosimetry calculation, measurement using a-Si 1200 EPID, and analysis. Adjustments were made to MLC Dynamic Leaf Gap (DLG) and MLC Transmission Factor (TF) to achieve the desired γ-pass rate.
Approximately 120 different plans were created, nearly 400 dose calculations were performed, and about 150 portal dosimetry measurements were made. Results were cross-validated using ArcCHECK and independently verified by three IROC Phantom studies.
Results:
The final mean γ-pass rates were high across different energy modes: 99.05±0.91% for 6X, 99.96±0.06% for 6X-FFF, 99.30±0.67% for 10X, 99.88±0.14% for 10X-FFF, and 99.40±0.45% for 16X (using γ-pass criteria: 2%/2mm).
The ratio of Mean TPS Dose to Mean Measured Dose in a user-defined ROI was close to 1.000 for all energy modes, indicating excellent dose agreement.
Portal dosimetry results for IROC Phantom studies showed high γ-pass rates: 98.73±0.72% for IMRT H/N Phantom, 99.90±0.17% for 4D Lung SBRT Phantom, and 99.87±0.23% for Prostate SBRT Phantom.
Conclusion: The study concluded that a-Si 1200 EPID portal dosimetry is an effective and robust tool for VMAT commissioning. It meets all necessary technical requirements such as spatial resolution, accuracy, response linearity, dynamic range, efficiency, and ease of use. It outperforms other dosimeters like ionization chambers, 2D/3D array detectors, TLDs, and EBT films and is particularly suitable for small centers operating under tight commissioning deadlines.