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Modeling for Total Skin Electron Treatments to Optimize the Beam Geometry for Dose Uniformity

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C Bosse

C Bosse*, D Defoor , D Stanley , J Roring , N Papanikolaou , S Stathakis , K Rasmussen , N Kirby , University of Texas HSC SA, San Antonio, TX


TU-RPM-GePD-T-3 (Tuesday, August 1, 2017) 3:45 PM - 4:15 PM Room: Therapy ePoster Lounge

Purpose: To find the optimal angle and source to surface distance (SSD) for dose uniformity when commissioning total skin electron (TSE) treatments.

Methods: Commissioning of TSE was completed for a Varian 21EX linear accelerator with the Stanford technique. This TSE implementation utilized a 9 MeV beam with a 1 mm aluminum accessory scattering foil and a 1.2 cm acrylic spoiler. First, dose measurements were taken with a parallel plate ionization chamber along a patient height of 175 cm for five selected SSDs. These measurements were utilized to interpolate a beam model for dose as a function of SSD and off-axis distance. From this beam model, dose uniformity could be calculated for all possible patient SSDs and gantry angles. Optically stimulated luminescent detectors (OSLD) and an in-house phantom were utilized to measure and validate three setups near the optimal beam geometry, along a patient height of 205 cm. After selecting the optimal treatment geometry, Task Group 51 was completed and then extended SSD and B values were characterized and validated with an end-to-end test.

Results: An SSD of 325 cm and beam angles of 20.5 degrees above and below lateral achieved the best dose uniformity over the treatment field. This setup produced a relative standard deviation of 3.8 percent over the treatment height with a maximum deviation of 8.3 percent, which agreed with the beam model predictions.

Conclusion: The TSE beam model enabled a global optimum to be found for the geometry and narrowed down the geometry validation measurements needed to only one SSD and three gantry angles. The final produced dose uniformity is superior to the Task Group 30 recommendations.

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