ESTRO 2020 Abstract Book

S802 ESTRO 2020

framework. APM substitutes the IDD with a sum of up to 13 Gaussian functions. This enables an efficient integration of lung degradation effects through analytical convolution with Gaussian modulation kernels while maintaining compatibility with matRad’s biological models. Proton and carbon ion treatment planning relies on validated base data against the Syngo TPS from the Heidelberg Ion Therapy Center (HIT). For helium ions, generic machine data was fitted to MC simulations. Each plan was optimized on both physical and RBE- weighted dose with similar objectives and dose per fraction. The plans were recalculated including heterogeneity effects induced by lung-tissue. The resulting dose distributions were compared to the uncorrected dose in terms of homogeneity D 5 -D 95 , dose coverage ΔD 95 , and dose difference in the planning target volume (PTV). Results In Figure 1, heterogeneity-corrected RBE-weighted dose distributions of one selected patient are shown for all particles, and the relative difference to the uncorrected dose distributions which reveals deviations up to 11.3% for helium ions. Figure 2 shows selected quality indicators for all patients inside the PTV. When heterogeneity is considered, the mean dose decreases inside the PTV. The correction yields a larger D5-D95 (less homogeneous) and a lower D95 (less dose coverage). This holds true for both physical and RBE- weighted dose, yet no systematic differences can be identified. However, the difference is larger for helium and carbon ions than for protons.

Conclusion For the facilitated degradation model based on an analytical convolution within a pencil beam algorithm, we did not observe large differences caused by degradation between physical and RBE-weighted dose. Yet, large local differences were observed. While their effect might be smaller than other uncertainties in particle lung treatments, the systematic nature may still require mitigation. This study shows that future analysis with MC simulations on treatment plans is worthwhile, especially in combination with 4D data to compare the magnitude of degradation to motion induced effects. PO-1491 What is best practice for using a mixed technique concept for hypofractionated RT of breast cancer? A. Venjakob 1 , M. Oertel 1 , H.T. Eich 1 , U. Haverkamp 1 1 Universitätsklinikum Münster, Klinik für Strahlentherapie - Radioonkologie, Münster, Germany Purpose or Objective The following analysis deals with the application of a mixed technique concept for the RT of breast cancer, which is a combination of static 3D-CRT and dynamic VMAT technique. In the following, the influence of its use on the dose distribution (PTV dose supply and OAR doses) is analyzed. The aim is to formulate a recommendation on the most ideal procedure. Material and Methods For each of five selected patients, 32 different mixed combinations (64 individual treatment plans) and a reference plan using conventional 3D-CRT technique were prepared. The hypofractionated radiotherapy of breast cancer is done in our institution with a total dose of 40.05 Gy in 15 daily fractions, 2.67 Gy per fraction. In treatment planning, the mixing ratio between the techniques was varied, 60:40 and 80:20 (3D-CRT:VMAT), respectively. Different energy and wedge constellations were used. The PTV dose supply was evaluated by indexes, whereby homogeneity and conformity (COV, CN, C ICRU, C DELTA, CI95, CI, CI HT) were evaluated. Doses of OAR (ipsilateral lung, heart, contralateral breast and spinal cord) served as further assessment criteria.