Beamlet based direct aperture optimization for MERT using a photon MLC

Henzen, Dominik; Manser, Peter; Frei, D.; Volken, W.; Neuenschwander, H.; Born, E; Joosten, A.; Lössl, Kristina; Aebersold, Daniel M.; Chatelain, Cécile; Stampanoni, Marco; Fix, M.K.

doi:10.1118/1.4901638

Cited by 19 publications

(27 citation statements)

References 67 publications

(60 reference statements)

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“…Since the robustness analysis for this work has been performed on dose distributions obtained from beamlet‐based apertures, an assessment of robust MBRT delivery will require a final MC recalculation and reweighting of each aperture will be necessary to obtain proper monitor units . However, this reoptimization does not significantly affect the conclusions of this work with respect to robustness.…”

Section: Discussionmentioning

confidence: 99%

“…The MC beam model was modified from a previously commissioned Varian Clinac 2100 EX model to match TrueBeam electron measured data. The phase space files were generated for a 40 × 40 cm

^{2}

field and manually split into 1 × 1 cm

^{2}

subfields to produce the electron beamlets …”

Section: Methodsmentioning

confidence: 99%

“…The robustness analysis is performed on dose distributions obtained from beamlet‐based apertures. For an analysis of robust MBRT delivery, which will be the topic of future work, a final MC recalculation and reweighting of each aperture will be necessary to obtain the monitor units for each aperture …”

Section: Methodsmentioning

confidence: 99%

“…However, routine clinical adoption of advanced electron techniques such as modulated electron radiation therapy (MERT) has yet to become reality, in part due to logistical issues such as unwieldy electron‐specific accessories (e.g., applicators, cerrobend cutouts, etc) and the limited dosimetric gains demonstrated in the literature. Typically, the superior normal tissue sparing of MERT plans comes at the expense of target coverage and homogeneity when compared to photon IMRT plans …”

Section: Introductionmentioning

confidence: 99%

“…These issues were addressed in recent studies by demonstrating the viability of delivering modulated electron radiation therapy (MERT) with a photon multileaf collimator (MLC) to eliminate the accessory requirements (applicators, cut‐outs, electron‐specific MLC) of traditional electron RT . By delivering MERT plans with a standard photon MLC, the potential arises for seamless mixed electron–photon delivery without interrupting treatment delivery between modality changes.…”

Section: Introductionmentioning

confidence: 99%

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Robust mixed electron–photon radiation therapy optimization

2019

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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...

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Section: Discussionmentioning

confidence: 99%

“…The MC beam model was modified from a previously commissioned Varian Clinac 2100 EX model to match TrueBeam electron measured data. The phase space files were generated for a 40 × 40 cm

^{2}

field and manually split into 1 × 1 cm

^{2}

subfields to produce the electron beamlets …”

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Robust mixed electron–photon radiation therapy optimization

2019

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On mixed electron–photon radiation therapy optimization using the column generation approach

2017

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Purpose Despite considerable increase in the number of degrees of freedom handled by recent radiotherapy optimisation algorithms, treatments are still typically delivered using a single modality. Column generation is an iterative method for solving large optimisation problems. It is well suited for mixed‐modality (e.g., photon–electron) optimisation as the aperture shaping and modality selection problem can be solved rapidly, and the performance of the algorithm scales favourably with increasing degrees of freedom. We demonstrate that the column generation method applied to mixed photon–electron planning can efficiently generate treatment plans and investigate its behaviour under different aperture addition schemes. Materials and methods Column generation was applied to the problem of mixed‐modality treatment planning for a chest wall case and a leg sarcoma case. 6 MV beamlets (100 cm SAD) were generated for the photon components along with 5 energies for electron beamlets (6, 9, 12, 16 and 20 MeV), simulated as shortened‐SAD (80 cm) beams collimated with a photon MLC. For the chest wall case, IMRT‐only, modulated electron radiation therapy (MERT)‐only, and mixed electron–photon (MBRT) treatment plans were created using the same planning criteria. For the sarcoma case, MBRT and MERT plans were created to study the behaviour of the algorithm under two different sets of planning criteria designed to favour specific modalities. Finally, the efficiency and plan quality of four different aperture addition schemes was analysed by creating chest wall MBRT treatment plans which incorporate more than a single aperture per iteration of the column generation loop based on a heuristic aperture ranking scheme. Results MBRT plans produced superior target coverage and homogeneity relative to IMRT and MERT plans created using the same optimisation criteria, all the while preserving the normal tissue‐sparing advantages of electron therapy. Adjusting the planning criteria to favour a specific modality in the sarcoma case resulted in the algorithm correctly emphasizing the appropriate modality. As expected, adding a single aperture per iteration yielded the lowest (best) cost function value per aperture included in the treatment plan. However, a greedier scheme was able to converge to approximately the same cost function after 125 apertures in one third of the running time. Electron apertures were on average 50–100% larger than photon apertures for all aperture addition schemes. The distribution of intensities among the available modalities followed a similar trend for all schemes, with the dominant modalities being 6 MV photons along with 6, 9 and 20 MeV electrons. Conclusion The column generation method applied to mixed modality treatment planning was able to produce clinically realistic treatment plans and combined the advantages of photon and electron radiotherapy. The running time of the algorithm depended heavily on the choice of mixing scheme. Adding the highest ranked aperture for each modality provided the best trade‐off b...

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