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Robustness Study of Passive-Scattering Proton Therapy in Lung: Is Range and Setup Uncertainty Calculation On the Initial CT Enough to Predict the Plan Robustness?

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X Ding

X Ding*, J Dormer , O Kenton , H Liu , C Simone , T Solberg , L Lin , University of Pennsylvania, Philadelphia, PA


SU-E-T-287 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Plan robustness of the passive-scattering proton therapy treatment of lung tumors has been studied previously using combined uncertainties of 3.5% in CT number and 3 mm geometric shifts. In this study, we investigate whether this method is sufficient to predict proton plan robustness by comparing to plans performed on weekly verification CT scans.

Ten lung cancer patients treated with passive-scattering proton therapy were randomly selected. All plans were prescribed 6660cGy in 37 fractions. Each initial plan was calculated using +/- 3.5% range and +/-0.3cm setup uncertainty in x, y and z directions in Eclipse TPS(Method-A). Throughout the treatment course, patients received weekly verification CT scans to assess the daily treatment variation(Method-B). After contours and imaging registrations are verified by the physician, the initial plan with the same beamline and compensator was mapped into the verification CT. Dose volume histograms (DVH) were evaluated for robustness study.

Differences are observed between method A and B in terms of iCTV coverage and lung dose. Method-A shows all the iCTV D95 are within +/- 1% difference, while 20% of cases fall outside +/-1% range in Method-B. In the worst case scenario(WCS), the iCTV D95 is reduced by 2.5%. All lung V5 and V20 are within +/-5% in Method-A while 15% of V5 and 10% of V20 fall outside of +/-5% in Method-B. In the WCS, Lung V5 increased by 15% and V20 increased by 9%. Method A and B show good agreement with regard to cord maximum and Esophagus mean dose.

This study suggests that using range and setup uncertainty calculation (+/-3.5% and +/-3mm) may not be sufficient to predict the WCS. In the absence of regular verification scans, expanding the conventional uncertainty parameters(e.g., to +/-3.5% and +/-4mm) may be needed to better reflect plan actual robustness.

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