ESTRO 2020 Abstract book

S869 ESTRO 2020

sample unpaired Mann-Whitney U-test(α=0.05).The association between MHD and mean LV and LAD doses was determined using linear regression. Results The study cohort consisted of 117 3DCRT patients (68 left(L)-sided and 49 right(R)-sided BC) and 114 IMRT patients (76L-sided and 38R-sided). IMRT was offered to those patients whose treatments did not meet risk organ restriction doses with 3DCRT. Patient characteristics and dosimetric parameters are shown in Table1. For the L-BC patients we observe that in IMRT group, despite having a higher MHD the rest of the parameters are lower. In the R- BC patients this correlation is similar. As we can see in Figure1, in IMRT groups the correlation between MHD and the rest of the cardiac parameters was not as strong as in 3DCRT ipsilateral group, and the data dispersion was greater, especially in LAD.

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 IMRT in left-BC compared to 3DCRT decreases doses of contoured cardiac substructures based on MHD as a reference. Correlation between MHD and other cardiac dosimetric parameters is weaker in IMRT than in 3DCRT, as is the ability to predict doses in cardiac substructures from MHD, especially LAD.