ESTRO 2020 Abstract book

S852 ESTRO 2020

The minimum dose requirement of the PTV (Dmean = 40.05 Gy (100%), D95% = 38.05 Gy (95%), D2% = 42.85 Gy (107%)) is not achieved by a conventional reference plan and all mixed constellations. PTV dose coverage problems mainly occur in 60:40 and primary constellations with use of 15 MV photons. Both mixing ratios are far superior to the conventional technique in PTV dose coverage, conformity and homogeneity. Combinations of 80:20, whose 3D-CRT base plan consists of only 6 MV photons (wedge constellation and VMAT energy play a secondary role), provide the best possible PTV dose coverage and homogeneity. A higher weighting of the VMAT technique using the ratio 60:40 can only slightly improve the conformity. Regarding mixed techniques, the ratio of 80:20 guaranteed better OAR protection. Anyhow, conventional 3D-CRT can assert itself in terms of OAR protection versus mixing techniques. Only in exceptional cases do combinations of 80:20 provide more optimal values (especially regarding heart and contralateral breast).

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. Results

Conclusion Compared to the conventional tangential field arrangement, the mixing technique can generate more conformal and homogeneous dose distributions and reduce underdoses as well as dose maximums. The ratio 80:20 is superior to the 60:40 and conventional 3D-CRT technique in PTV dose coverage, homogeneity and OAR doses (some cases). When using the mixing concept, a ratio 80:20 and constellations with the sole use of 6 MV photons as the 3D- CRT base plan should be preferred. Base plan wedge constellation and VMAT energy are of secondary importance. PO-1492 VMAT modalities scenarios in ESTRO-ACROP consensus guidelines postmastectomy implants radiotherapy G. Lazzari 1 , M.G. Monis 1 , G. Porrazzo 1 , M. Clarizio 1 , D. Becci 2 , A. Bruno 2 , G. De Zisa 2 , D. Mola 2 , G. Silvano 1 1 Azienda Ospedaliera SS. Annunziata Presidio Osped, Radiology, Taranto, Italy ; 2 Azienda Ospedaliera SS. Annunziata Presidio Osped, Fisica Sanitaria Department, Taranto, Italy