ESTRO 2020 Abstract Book

S820 ESTRO 2020

PO-1517 Development and validation of a 3D dose calculation software for neutron radiotherapy L. Sommer 1,2,3 , S.E. Combs 1,4 , H. Breitkreutz 3 , T. Chemnitz 3 , J.J. Wilkens 1,2 1 School of Medicine and Klinikum rechts der Isar - Technical University of Munich TUM, Department of Radiation Oncology, Munich, Germany ; 2 Technical University of Munich TUM, Physics Department, Munich, Germany ; 3 Research Neutron Source Heinz Maier- Leibnitz FRM II - Technical University of Munich TUM, Medical Application Facility MedApp, Garching, Germany ; 4 Helmholtz Zentrum München, Institute for Innovative Radiotherapy iRT, Neuherberg, Germany Purpose or Objective Around the world, only a few remaining fast neutron therapy (FNT) facilities exist. Accordingly, the number of patients treated with neutron therapy is rather small. However, keeping in mind their high RBE, high LET and the low OER for neutron radiation, the potential of neutrons especially for superficial tumors in palliative radiotherapy settings is worthwhile to consider. The purpose of this work is to develop and validate a 3D dose calculation and treatment-planning environment oriented at state of the art photon and charged particle dose calculation techniques for a fission neutron source. Material and Methods In order to realize interaction simulations of neutrons within a patient, the MATLAB based research treatment planning software matRad (www.matrad.org) was adopted and – while leaving its IMRT planning infrastructure unchanged - its dose calculation engine was replaced by a Monte Carlo approach. Here, the Monte Carlo code MCNP6 is used to calculate the dose distribution on a voxelized patient geometry for every neutron ray contributing to the radiation field emerging from an area source and shaped by a multi leaf collimator. Information on the elemental tissue composition and mass density in the calculation volume are obtained from patient CT scans. Since not only neutrons but also photons are generated during the fission process, their contribution to the total patient dose is non-negligible. Therefore, the presented Monte Carlo dose engine also includes photon and electron interaction and dose contribution in the simulation volume. So the dose simulation is possible for neutrons and both for (primary and secondary) photons and for secondary electrons propagating in the patient. In a subsequent step, the generated dose influence matrix is optimized in matRad. In addition to this inverse planning approach (IMRT), it is also possible to calculate whole treatment field dose distributions on patients and skip the IMRT optimization procedure in matRad. This follows the objective to enable retrospective calculations of neutron dose distributions for FNT performed in the past. In addition, a modification of KERMA values for neutron interaction in tissue by tabulated RBE values allows the consideration of neutron RBE in the treatment planning process and the retrospective evaluation based on RBE- weighted dose. Results First validation measurements of relative neutron depth dose curves in water have shown good agreement with calculated neutron dose distributions in a virtual soft tissue phantom up to a depth of 12 cm in the water phantom (see figure).

Conclusion The described set-up allows the user to perform treatment plan calculations for fission neutrons including an individual photon source for Monte Carlo IMRT calculations with MCNP for research purposes. Acknowledgement: This work was supported by the German Research Foundation (DFG) within the Research Training Group GRK 2274. PO-1518 Total Body Irradiation with VMAT: pre- implementation anthropomorphic phantom study J. Lencart 1,2 , M.D.F. Borges 1 1 Instituto Português de Oncologia do Porto Francisco Gentil- EPE, Medical Physics, Porto, Portugal ; 2 IPO-Porto Research Center, Medical Physics Radiobiology and Radiattion Protection Research Group, Porto, Portugal Purpose or Objective Total Body Irradiation (TBI) as well as Total Marrow Irradiation (TMI) are among the election conditioning regimens prior to bone marrow or allogenic stem cell transplantation. As such, it is our hospital’s intention to start using this technique. Although conventional irradiation with two parallel opposed fields is accepted as a standard, the use of VMAT is being considered by some centers. Our aim is to carry out a feasibility study of the use of VMAT for TBI, considering the installed technical capabilities. Material and Methods A Rando Alderson female anthropomorphic phantom was CT scanned in Head First Supine (HFS) position. As a first approach, the PTV was obtained considering the whole body except the lungs, with a 2mm crop to the body surface, from the head to mid-thigh. A VMAT plan containing 8 hemi field full arcs distributed over 3 isocentres was generated (Varian Eclipse 13.5). For optimization purposes, a dose of 13.2Gy (8 fractions) to the PTV (95% of the dose covers 95% of the PTV) and mean dose of 10Gy to the lungs, maintaining dose homogeneity elsewhere, was considered. Field length and collimator rotation of each hemi-field full arc were adjusted accordingly to the PTV length. EBT3 gafchromic films were used to measure the dose distribution during the irradiation and compare it with the calculated dose distribution in specific axial plans (base of the orbits; neck, torax at supra-carinal level and abdominal at umbilicus level). Results PTV dose homogeneity is reasonable (mean 14.3Gy and median 14.6Gy) and the mean dose to the lungs 9.8Gy, fulfilling the dose tolerance published. The homogeneity of the dose distribution in all sections evaluated was confirmed by the film dosimetry and at the lungs level the film confirms the predicted dose distribution.