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Evaluation of Secondary Dose Calculation Software: Ability to Detect Systematic Errors in the Treatment Planning System Beam Model

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J Wong

J Wong*, W Warren, K Homann, Houston Methodist Hospital, Houston, TX


SU-I-GPD-T-227 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this study was to evaluate a commercial secondary dose calculation software’s (SDCS) ability to detect systematic errors in a treatment planning system (TPS) beam model for model validation and treatment plan approval.

Methods: Artificial changes to the TPS beam model introduced errors to patient plans. 12 cases were analyzed with the SDCS to determine effects of the following: 1. Incorrect TPS settings for leaf-end modeling regarding (a) MLC offset table, and (b) leaf tip radius, 2. Incorrect TPS settings to account for tongue-and-groove effects, 3. Dose gradient errors due to inaccurate volume-averaged dose profiles, 4. Inherent dose gradient errors in the TPS beam model/algorithm, and 5. TPS underestimation of dose for narrow MLC segments.

Results: Plans were evaluated with the SDCS for gamma dose criteria, mean target dose, dose coverage, dose volume histogram, and profiles. Study 1(a) resulted in no plans failing. However, a decrease in the mean target dose and 90% dose coverage can been seen throughout all plans. Study 1(b) showed 10 plans failing at 2%/2mm when the leaf tip radius was increased by 6 cm. Study 2 showed minimal differences in passing rates when not accounting for the tongue-and-groove effects. However, an increase in the mean target dose and 90% dose coverage is seen for all but one plan. Study 3 shows 2 plans failed at 2%/2mm and 10 plans with decreased gamma values. Study 4 reviewed the original plans which revealed an underlying dose pattern corresponding to overestimation and underestimation of dose throughout. Study 5 further attributed dose differences to narrow MLC segments especially in highly-modulated cases.

Conclusion: This study verified errors in commissioning measurement and beam modeling can be detected with the SDCS. However, large modeling errors or tighter tolerances are needed before clinical failures can be detected.

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