Room: AAPM ePoster Library
Purpose: To investigate the impact of sampling time on the performance of a commercial plastic scintillation dosimeter (PSD) system in profile scanning of small fields in clinical photon beams.
Methods: Beam profile measurements of a 6 MV photon beam delivered by a Varian 21Ex linear accelerator were performed using an Exradin W2 scintillation detector and MAX SD electrometer system at different field sizes and depths. Cross-line beam profiles were measured using the Standard Imaging DoseView 3D scanning water tank with a scanning resolution of 1mm. Beam profile scans were performed with field sizes of 0.5x0.5cm² and 1x1cm² and at depths of 1.5cm, 5cm, 10cm, and 20cm. For each given field size and depth, profile scans were measured using sampling times ranging from 100ms to 2000ms. Profiles were compared to averaged referenced profiles performed with the recommended 1000ms sampling time at each corresponding field size and depth using 1-D gamma analysis. The minimum percent dose difference (DD) and distance to agreement (DTA) criteria were determined to achieve gamma pass rates of 95% and 99% reproducibility of the 1000ms scan.
Results: The gamma criteria used were based on a self test of the 1000 ms scan to itself. When the gamma criteria were set to 1.0% and 0.50mm, all profiles for all depths achieved 99% gamma pass rates for sampling times of 700ms or greater. When the gamma criteria were set to 0.75% and 0.25mm, all profiles for all depths achieved 99% gamma pass rates for sampling times of 700ms or greater.
Conclusion: When taking small field beam profile measurements with the Exradin W2, the sampling time can be smaller than the recommended sampling time of 1000ms while retaining the necessary accuracy of the measurements. This is especially important when taking multiple scans to reduce overall measurement time.