title: Forward treatment planning for modulated electron radiotherapy (MERT) employing Monte Carlo methods.
abstract: OBJECTIVE
  This paper describes the development of a forward planning process for modulated electron radiotherapy (MERT). The approach is based on a previously developed electron beam model used to calculate dose distributions of electron beams shaped by a photon multi leaf collimator (pMLC).
  
  
  METHODS
  As the electron beam model has already been implemented into the Swiss Monte Carlo Plan environment, the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) can be included in the planning process for MERT. In a first step, CT data are imported into Eclipse and a pMLC shaped electron beam is set up. This initial electron beam is then divided into segments, with the electron energy in each segment chosen according to the distal depth of the planning target volume (PTV) in beam direction. In order to improve the homogeneity of the dose distribution in the PTV, a feathering process (Gaussian edge feathering) is launched, which results in a number of feathered segments. For each of these segments a dose calculation is performed employing the in-house developed electron beam model along with the macro Monte Carlo dose calculation algorithm. Finally, an automated weight optimization of all segments is carried out and the total dose distribution is read back into Eclipse for display and evaluation. One academic and two clinical situations are investigated for possible benefits of MERT treatment compared to standard treatments performed in our clinics and treatment with a bolus electron conformal (BolusECT) method.
  
  
  RESULTS
  The MERT treatment plan of the academic case was superior to the standard single segment electron treatment plan in terms of organs at risk (OAR) sparing. Further, a comparison between an unfeathered and a feathered MERT plan showed better PTV coverage and homogeneity for the feathered plan, with V95% increased from 90% to 96% and V107% decreased from 8% to nearly 0%. For a clinical breast boost irradiation, the MERT plan led to a similar homogeneity in the PTV compared to the standard treatment plan while the mean body dose was lower for the MERT plan. Regarding the second clinical case, a whole breast treatment, MERT resulted in a reduction of the lung volume receiving more than 45% of the prescribed dose when compared to the standard plan. On the other hand, the MERT plan leads to a larger low-dose lung volume and a degraded dose homogeneity in the PTV. For the clinical cases evaluated in this work, treatment plans using the BolusECT technique resulted in a more homogenous PTV and CTV coverage but higher doses to the OARs than the MERT plans.
  
  
  CONCLUSIONS
  MERT treatments were successfully planned for phantom and clinical cases, applying a newly developed intuitive and efficient forward planning strategy that employs a MC based electron beam model for pMLC shaped electron beams. It is shown that MERT can lead to a dose reduction in OARs compared to other methods. The process of feathering MERT segments results in an improvement of the dose homogeneity in the PTV.
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Henzen
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Manser
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Frei
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: W
authors.initials: W
authors.lastname: Volken
authors.affiliation: Clinic for Radiation-Oncology, Lindenhofspital Bern, CH-3012 Berne, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Neuenschwander
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: E J
authors.initials: EJ
authors.lastname: Born
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: K
authors.initials: K
authors.lastname: Lössl
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: D M
authors.initials: DM
authors.lastname: Aebersold
authors.affiliation: Institute for Biomedical Engineering, ETH Zürich and Paul Scherrer Institut, CH-5234 Villigen, Switzerland.
authors.forename: M F M
authors.initials: MF
authors.lastname: Stampanoni
authors.affiliation: Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, University of Bern, CH-3010 Berne, Switzerland.
authors.forename: M K
authors.initials: MK
authors.lastname: Fix
mesh_terms: D000465:Algorithms; D001943:Breast Neoplasms / Q000532:radiotherapy*; D002138:Calibration; D004583:Electrons; D005260:Female; D006801:Humans; D009010:Monte Carlo Method; D016011:Normal Distribution; D010315:Particle Accelerators; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D014057:Tomography, X-Ray Computed