Robust mixed electron–photon radiation therapy optimization

Abstract

Purpose Mixed beam electron–photon radiation therapy (MBRT) is an emerging technique that has the potential to reduce dose to normal tissue while improving target coverage for cancer sites with superficial tumors. Advances in optimization algorithms and robotic linear accelerators have made the creation and delivery of complex MBRT plans realistic without the need for special additional collimators, devices, or resetup of the patient. However, no study has been performed on the robustness of MBRT dose distributions to patient setup errors. Intensity‐modulated delivery of other charged particles such as protons have been shown to require robust planning techniques to maintain adequate target coverage under positioning errors. We therefore assess the sensitivity of MBRT treatment plans to positioning uncertainties when created under the traditional planning target volume (PTV)‐based planning paradigm and present a novel optimization model for the creation of robust MBRT plans. Methods The column generation method was applied to robust MBRT treatment planning by deriving the pricing problem for stochastic and “worst case” minimax optimization models, two common formulations of robustness. Robust treatment plans were created for two patient cases representative of the cancer sites which stand to benefit from MBRT: soft tissue sarcoma (STS) irradiation and chest wall irradiation with deep‐seated internal mammary, axillary, and supraclavicular nodes (CW‐N). For both patient cases, beamlet dose distributions for electrons and photons were generated for positioning shifts in six directions, ±5mm(xfalse^,yfalse^,zfalse^) in addition to a nominal unshifted scenario, for a total of seven sets of beamlets. Robust plans were created by specifying dose coverage constraints to the clinical target volume (CTV), as opposed to the PTV. Comparisons were performed against traditional PTV‐based plans created with a single set of unshifted beamlets. Results The dose distributions of traditional PTV‐based MBRT plans showed significant degradation in target coverage homogeneity when patient positioning errors were considered. For both cancer sites, cold spots below 95% and hot spots above 108% of the prescription dose appeared within the CTV when shifting the patient by 5 mm, corresponding to the margin added to the CTV to form the PTV. In contrast, CTV‐based robust plans created with the new optimization model maintained target coverage within the 95%–108% limits, for all positioning errors. Conclusion The quality of MBRT treatment plans created using a traditional PTV‐based optimization model was highly sensitive to patient positioning errors. For both patient cases, positioning errors resulted in perturbations to the nominal dose distributions which would have rendered PTV‐based plans clinically unacceptable. In contrast, CTV‐based robust plans were able to maintain adequate target coverage under all positioning error scenarios considered. We therefore conclude that to ensure the fidelity of the dose distribution delivered to the pat...

Ion chamber and film‐based quality assurance of mixed electron‐photon radiation therapy

“…Although MBRT plans in this study were not robustly optimized, Renaud et al (2019) 26 have shown that robust MBRT plans are necessary for deliveries of mixed electron‐photon modalities in realistic clinical conditions. This however raises the question of how to perform QA on the robustness of a plan.…”

“…The aim of this work was to confirm that the treatment plans produced by our MBRT treatment planning algorithm 16,26 could be delivered and measured accurately on a Varian TrueBeam accelerator. In addition, we sought to develop the framework required for a potential patient-specific QA protocol.…”

“…In previous work, we have demonstrated PTV‐based and CTV‐based robust treatment planning algorithms to produce MBRT plans from beamlet‐based apertures 16,26 . However, beamlet‐based apertures require a final MC recalculation to account for the effects of MLC leaves and jaw position on the dose distribution and the relative output of each aperture.…”

“…In previous work, we have demonstrated PTV-based and CTV-based robust treatment planning algorithms to produce MBRT plans from beamlet-based apertures. 16,26 However, beamlet-based apertures require a final MC recalculation to account for the effects of MLC leaves and jaw position on the dose distribution and the relative output of each aperture. In this work, each aperture of the treatment plans created for measurement purposes was recalculated using a validated MC beam model, and the MC dose distributions were renormalized from dose per primary particle to dose per monitor unit.…”

Joint Optimization of Photon–Carbon Ion Treatments for Glioblastoma

“…Full-fledged simultaneous optimization of mixed modality radiation treatments has been exercised for electron-photon combinations. [14][15][16] Fabiano et al 17 considered the combination of a horizontal fixed proton beamline combined with photons, and Kueng et al 18 consider triple modality photon −electron−proton treatments. In addition, several authors considered proton-photon treatments in which some fractions are delivered with protons and others with photons.…”