(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: aim of this study was to develop a method to identify patient-specific robust proton beam angles for lung tumor irradiation by investigating the association between water equivalent thickness (WET) variation and inter-fraction motion-induced target dose degradation.
Methods: 3-dimensional computed tomography (3D-CT) images, the impact of WET variations on target dose coverage for a series of coplanar proton beams was evaluated for four lung cancer patients. Using ray tracing, WET maps, or WET baseline, were estimated for the internal target volume (ITV) at every 5° gantry interval in the axial plane. After calculating the WET baseline, the planning CT was shifted 5 mm in each anterior-posterior (AP), superior-inferior (SI), and left-right (LR) direction, yielding a total of 6 shifted CTs, and differential WET maps between the planning CT and each shifted CT were calculated. Target dose differences were associated with the average WET change between the original planning CT and the shifted CTs for all beam rays across the 360° gantry rotation angle. Target and OAR dose metrics in the ?WET-guided plans were compared with those of the clinical plans.
Results: WET variation maps showed areas of both high and low WET variations, with overall similar patterns yet individual differences reflecting differences in tumor position. The coplanar plans showed a strong association between WET changes and ITV dose reductions for all 4 patients investigated in this study. Target dose coverage was more stable with the ?WET-guided plan while OAR doses were comparable to the clinical plan.
Conclusion: WET variation maps have been used in this pilot study to identify proton beam angles that are either sensitive or robust to WET changes in proton passive scattering. This work demonstrates the feasibility of using WET variation maps to assist the planner in inter-fraction motion-robust proton beam angle selection.
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