Comparison of Robust Optimization and PTV Margins for Mixed-Beam Radiotherapy
E Heath1*, S Mueller2, G Guyer2, O Elicin2, D Aebersold2, M Fix2, P Manser2, (1) Carleton Univ, Ottawa, ON, CA, (2) Inselspital, Bern University Hospital and University Of Bern, Bern, BE, CH
Presentations
TH-D-TRACK 5-2 (Thursday, 7/29/2021) 2:00 PM - 3:00 PM [Eastern Time (GMT-4)]
Purpose: Mixed-beam radiotherapy (MBRT) using intensity modulated electron and photon beams collimated by the photon MLC has been shown to give superior plan quality in the treatment of tumours with a superficial component. Conventional PTV margins may not be effective at compensating for setup errors in MBRT. We previously developed and validated a framework for robust optimization of deliverable MBRT plans. The aim of the current work is to apply this framework to clinical patient cases and to compare with PTV-based plans.
Methods: The framework uses a hybrid direct aperture optimization approach which combines simulated annealing and column generation algorithms. To make plans robust against setup errors, the optimization minimizes the expectation value of the objective function evaluated across all specified setup error scenarios under simultaneous consideration of both beam modalities. Robust-optimized and PTV-based MBRT plans were designed on three clinical cases consisting of two head and neck cases and one brain case. The PTV-based plans used a 5 mm isotropic PTV margin while the robust plans considered ±5 mm systematic translational setup errors along the principal axes (SI, LR, AP). For each plan, dose distributions were calculated for 5 mm isocenter shifts along the principal axes and were compared in terms of target coverage and OAR sparing.
Results: The robust optimized plans respected the institutional planning guidelines for all considered setup error scenarios while the PTV-based plans had clinically unacceptable hotspots in the CTV and exceeded OAR tolerances for many of the scenarios. Robust optimized plans achieved an 18% increase in the CTV dose homogeneity for the head and neck cases and, on average, a 6% reduction in Dmean and 15% reduction in D2% for the OARs in all patient cases.
Conclusion: Robust optimization is a promising alternative to conventional PTV margins to account for setup uncertainties in MBRT.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by grant 200021_185366 of the Swiss National Science Foundation and Varian Medical Systems.
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