ESTRO 36 Abstract Book

S133 ESTRO 36 2017 _______________________________________________________________________________________________

Conclusion A unique feature of strong inline magnetic fields is that they act to reduce lateral electron scatter in lower density mediums such as lung tissue. This work has demonstrated this experimentally for the first time. Clinically, such scenarios will arise in inline MRI-linac systems for treatment of small lung tumors. OC-0261 Monte Carlo Correction Factors for MRgRT Reference Dosimetry S. Pojtinger 1 , O. Dohm 2 , D. Thorwarth 1 1 University Hospital Tübingen, Department of RadiationOncology - Section for Biomedical Physics, Tübingen, Ger many 2 University Hospital Tübingen, Department of Radiation Oncology - Division for Medical Physics, Tübingen, Germany Purpose or Objective Magnetic-Resonance guided Radiotherapy (MRgRT) is a new technique providing real-time MR-Im aging during Radiotherapy (RT). Caused by the strong magnetic field, trajectories of charged particles change, w hich results in a different deposited dose in the ionization chamber. Hence, a new chamber-specific correction factor k B , taking into account the magnetic effect, has to be introduced. In this work k B is determined for seven commercially available chambers by Monte-Carlo (MC) simulations. Material and Methods Simulations were made using the MC package EGSnrc. Chambers were placed in 10cm depth, inside of a 30x30x20cm water phantom and dose was scored for magnetic field strengths ranging from 0 to 5T in steps of 0.5T. The orientation of chambers was perpendicular to the central axis of the beam for thimble type chambers and axial for plane-parallel chambers (see fig. 1). The magnetic field was oriented perpendicular to the central axis of the beam. Simulated chambers were PTW 30013, PTW 30015, PTW 34045 Advanced Markus, PTW 23343 Markus, PTW 34001 Roos (PTW Freiburg), NE2571 (Phoenix Dosimetry) and NACP02 (Scanditronix). Modeling the PTW 30013 was based on plans provided by manufacturer as well as on a µCT scan. For all other chambers, geometries and material information were used that have been provided and used in publications by Wulff et al [1]. Incoming particles were simulated in 4 different ways: 1. Commercial Elekta 6MV FFF accelerator has been modeled as part of the MC simulation (Elekta 6MV FFF(Beam Sim)) A photon spectrum was calculated from an earlier simulation of a commercial Elekta 6MV FFF accelerator. This spectrum was used as an input for modeling the beam in form of photons diced from this distribution (Elekta 6MV FFF) 2. The beam was simulated by photons sampled from a photon spectrum that was calculated from an earlier simulation of a commercial Elekta 6MV accelerator with flattening filter (Elekta 6MV) Photons were generated from a spectrum for a 6MV accelerator, published by Mohan et al [2] (Mohan 6MV) Results Figure 1 shows the calculated dose D normalized to the dose without magnetic field D nb . This is the inverse of the correction factor k B that is given for all simulation methods (at 1.5T) in table 1. Overall, large variations were observed depending on the type of ionization chamber and the magnetic field strength. Though effects are stronger for plane-parallel chambers 4. 3.

one can find points of stable dose, e.g. k B

=0.9997(19) for

PTW 23343 Markus at 3.5T.

Conclusion If plane-parallel chambers are used, there is a strong change in measured dose, as variations can range up to 10%. In contrast, the correction for thimble type chambers is below 2%. The results demonstrate that reference dosimetry for MRgRT is possible, using the presented chambers, but measured dose must be corrected by the calculated factors k B . References: [1] Phys Med Biol. 2008 Jun 7;53(11):2823-36 [2] Med Phys. 1985 Sep-Oct;12(5):592-7 OC-0262 Implementation of patient specific QA for daily adaptive MR-guided radiation therapy M.A. Palacios 1 , T. Apicella 2 , D. Hoffmans 1 , T. R osario 1 , M. Admiraal 1 , I. Kawrakow 2 , J. Cuijpers 1 1 V U Medical Center, Radiation Oncology Department, Amsterdam, The Netherlands 2 ViewRay- Inc., Research & Development, Mountain View, USA Purpose or Objective To report on the successful implementation of a patient specific QA procedure for online treatment plan adaptation under MR guidance. Material and Methods The ViewRay MRIdian system was recently ins talled at our institution. It allows for a fast online treatment plan adaptation based on the daily MR image and real time monitoring of the anatomy of the patient during treatment delivery. To facilitate the clinical implementation of online treatment plan adaptation we developed an online dosimetric verification procedure that uses an independent MC dose calculation engine and automatically checks relevant planning parameters. The independent MC includes the MLC, couch, and imaging coils in the simulation and takes into account the magnetic field. It runs on a computer equipped with a 4-core Intel® Core™ i3-2100 CPU @ 3.10GHz and 8 GB of RAM. A 3%/3mm Gamma comparison with the dose distribution from the TPS is performed before accepting an adapted treatment