Electron modulated arc therapy for breast boost: development of a treatment planning process
PD-0978
Abstract
Electron modulated arc therapy for breast boost: development of a treatment planning process
Authors: Gian Guyer1, Silvan Mueller2, Daniel Frei2, Werner Volken2, Kristina Loessl2, Daniel M. Aebersold2, Peter Manser3, Michael K. Fix3
1Inselspital, University Hospital Bern and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland; 2Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland; 3 Inselspital, Bern University Hospital and University of Bern, Division of Medical Radiation Physics and Department of Radiation Oncology, Bern, Switzerland
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Purpose or Objective
Electron beam radiotherapy for tumor bed boost irradiation after whole breast irradiation is currently performed using applicators and cut-outs mounted on a C-arm linear accelerator. Replacing the applicators and cut-outs with the already installed photon multileaf collimator (pMLC) reduces the effort for electron therapy as no cut-out needs to be fabricated and no applicator has to be mounted. Furthermore, the pMLC facilitates modulated electron radiotherapy (MERT). Electron modulated arc therapy (EMAT) extends MERT by dynamic gantry rotation and dynamic pMLC movement during beam on with the potential to reduce delivery time. The aim of this work is to develop a treatment planning process for pMLC based EMAT for breast boost irradiation.
Material and Methods
To create EMAT plans, a gantry range is defined for all available electron beam energies. The electron arcs maintain a shortened source-surface distance (SSD) of 80 cm by synchronized table movement to reduce air scatter. A fluence map optimization is performed for the electron arcs and the arcs are ranked based on their relative contribution to the PTV mean dose. Next, a direct aperture optimization is performed with the two highest-ranked electron arcs, resulting in a two-arc EMAT plan. EMAT plans are created retrospectively for three clinically motivated cases: two left and one right breast boost. The treatment plan quality and estimated delivery time of the EMAT plans are compared to cut- out plans using a single static electron field and to retrospectively created MERT plans using one beam direction with an SSD of 80 cm.
Results
The dose homogeneity index ( (D2% - D98%)/D50% ) is on average improved by 2% when comparing the EMAT to the MERT plans and improved by 18% when comparing the EMAT to the cut-out plans. The mean dose to the ipsilateral lung is on average 2% lower for the EMAT plans compared to the MERT plans, but 3% higher compared to the cut-out plans. The mean dose to the heart is lower than 0.5% of the prescribed dose for all plans of all considered treatment techniques. The estimated delivery time is on average 1.1 min for the EMAT plans and 2.7 min for the MERT plans.
Conclusion
A treatment planning process for EMAT applied to breast boost irradiation was developed. Improvements in the delivery time and planning of EMAT over MERT further facilitate more efficient electron treatments.
This work was supported by grant 200021_185366 of the Swiss National Science Foundation and Varian Medical Systems.