Program Information
Assessing the Capabilities of An MU Model for Fields as Small as 2cm in a Passively Scattered Proton Beam
R Simpson*, A Ghebremedhin , I Gordon , B Patyal , Loma Linda Univ Medical Center, Loma Linda, CA
Presentations
SU-E-T-134 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:
To assess and expand the capabilities of the current MU model for a passively scattered proton beam. The expanded MU model can potentially be used to predict the dose/MU for fields smaller than 2cm in diameter and reduce time needed for physical calibrations.
Methods:
The current MU model accurately predicted the dose/MU for more than 800 fields when compared to physical patient calibrations. Three different ion chambers were used in a Plastic Water phantom for physical measurements: T1, PIN, and A-16. The original MU model predicted output for fields that were affected by the bolus gap factor (BGF) and nozzle extension factor (NEF). As the system was tested for smaller treatment fields, the mod wheel dependent field size factor (MWDFSF) had to be included to describe the changes observed in treatment fields smaller than 3cm. The expanded model used Clarkson integration to determine the appropriate value for each factor (field size factor (FSF), BGF, NEF, and MWDFSF), to accurately predict the dose/MU for fields smaller than 2.5cm in effective diameter.
Results:
The expanded MU model demonstrated agreement better than 2% for more than 800 physical calibrations that were tested. The minimum tested fields were 1.7cm effective diameter for 149MeV and 2.4cm effective diameter for 186MeV. The inclusion of Clarkson integration into the MU model enabled accurate prediction of the dose/MU for very small and irregularly shaped treatment fields.
Conclusion:
The MU model accurately predicted the dose/MU for a wide range of treatment fields used in the clinic. The original MU model has been refined using factors that were problematic to accurately predict the dose/MU: the BGF, NEF, and MWDFSF. The MU model has minimized the time for determining dose/MU and reduced the time needed for physical calibrations, improving the efficiency of the patient treatment process.
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