ESTRO 2020 Abstract book

S610 ESTRO 2020

the target volume. The gross tumor volume (GTV), serving as boost, is substantially smaller than the clinical target volumes (CTV) to which a lower dose is prescribed. At present, it is unclear whether a margin leading to the planning target volume (PTV) is required for both the GTV and CTV. Therefore, we evaluated the possibility to deliver a SIB with a highly conformal photon and proton technique, and the resulting plan robustness. Material and Methods In CT scans of four patients a GTV (macroscopic tumor), a CTV (microscopic tumor extension and elective volume), PTV with an isotropic 5mm margin around the GTV (PTV GTV ) and CTV (PTV CTV ) as well as the OARs were delineated. Two volumetric modulated arc therapy (VMAT) plans each with a dose prescription of 51Gy to the PTV CTV and 66Gy to a) the GTV (VMAT GTV ) or b) the PTV GTV (VMAT PTV_GTV ) were optimized (RayStation V5.99, RaySearch), respectively. Moreover, a robustly optimized multi-field IMPT [rMFO; robustness parameters for setup errors (x,y,z=±5mm) and range uncertainties (±4.5%)] plan each was calculated with a radiobiological equivalent (RBE=1.1) dose prescription of 51Gy(RBE) to the CTV and of 66Gy(RBE) to the GTV. For both techniques, at least 95% of the target volume was to be covered with more than 95% of the respective dose prescription while not exceeding a near-maximum (D 2% ) of 107% of the prescribed dose. Local and QUANTEC dose constraints for OARs were considered. Robustness analyses of the target coverage were performed for each treatment plan by perturbing the dose in 20 scenarios with a setup error of x,y or z=±5mm (VMAT, rMFO) and a range uncertainty of 0% or ±4.5% (rMFO). Results The target coverage of the GTV and CTV was fulfilled for each treatment plan. While the VMAT PTV_GTV and the rMFO treatment plan kept the robustness requirement to GTV and CTV in each scenario, the VMAT GTV showed a significant underdosage of the GTV when performing a robustness analysis (Fig. 1). In the VMAT PTV_GTV plans, the volume of the stomach and bowel, receiving a dose of 50Gy (V 50Gy ), were increased by mean 37.5% and 33.2% in comparison to the VMAT GTV plans. Although robust optimization was applied in the rMFO plans, the V 50Gy was superior for both OAR (stomach: -34.4%, bowel: -10.9%) in comparison to the VMAT GTV treatment plans. Conclusion A PTV margin around the GTV is recommended to increase treatment plan robustness and thus avoid underdosage of the boost, despite higher doses to OARs. This also applies to dose-painting techniques both on contours or voxel- based.

history of oligometastatic disease (OMD) and how SBRT may impact in the transition to the polymetastatic disease (PMD). Material and Methods 97 liver metastases in 61 oligometastatic patients were treated with SBRT at our Institution. Primary tumor histology was: colorectal (29.5%), gastrointestinal (28%), breast (16.5%), lung (8%), gynecological (5%) and other (13%). Twenty patients (32.7%) had synchronous oligometastases, 41 (67.3%) metachronous oligometastases. Median number of treated metastases was 2 (range 1-5). Time to PMD (ttPMD) was defined as the time from the SBRT to the occurrence of >5 new metastases. Results The median follow-up was 24 months. Median PFS was 7 months (range 3-12 months). Patients with 1 metastasis had longer median PFS as compared to those with 2-3 and 4-5 metastases (14.7 months versus 5.3 months versus 6.5 months; p=0.041) At the last follow-up 49/61 patients (80%) progressed, 21 of which (34.4%) again as oligometastatic and 28 (45.9%) as polymetastatic. Median ttPMD was 11 months (range 6-15 months). Median overall survival (OS) was 24 months (range 16-29 months). Patients with further OMD after SBRT had longer median OS as compared to patients with PMD (21 months versus 15 months; p=0.007). One- and 2-year local progression-free survival were 71.8% and 59.4%, respectively, median was not reached. At the ROC curve analysis a lesion diameter ≤29 mm correlated to higher LPFS (AUC 0.62). At the univariate analysis OS was significantly longer for patients with ≤4 metastases (p=0.0). No cases of radiation induced liver disease or grade ≥3 toxicity occurred. Conclusion In the setting of oligometastatic disease, SBRT is able to delay the transition to PMD. A proportion of patients relapse as oligometastatic and can be eventually evaluated for a further SBRT course. Interestingly, those patients retain a survival benefit as compared to those who had PMD. Further studies are needed to explore the role of SBRT in OMD and to identify treatment strategies able to maintain the oligometastatic state. PO-1063 Do we need a PTV around the boost in simultaneously integrated boost approaches of abdominal tumors? S. Stefanowicz 1,2,3 , S. Zschaeck 4,5 , E.G.C. Troost 1,2,3,6,7 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany ; 2 Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Department of Radiotherapy and Radiation Oncology, Dresden, Germany ; 3 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology – OncoRay, Dresden, Germany ; 4 Charité Universitätsmedizin Berlin, Department of Radiation Oncology, Berlin, Germany ; 5 Berlin Institute of Health BIH, Anna-Louisa-Karsch 2, Berlin, Germany ; 6 German Cancer Consortium - DKTK, Partner Site Dresden and German Cancer Research Center - DKFZ, Heidelberg, Germany ; 7 National Center for Tumor Diseases - NCT, Partner Site Dresden- Germany: German Cancer Research Center DKFZ- Heidelberg- Germany- Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Dresden- Germany- Helmholtz, Purpose or Objective In locally advanced pancreatic cancer patients, radiosensitive organs at risk (OARs) encompass the target volume hampering the delivery of sufficiently high dose. Simultaneously integrated boost (SIB) approaches using intensity-modulated radiotherapy techniques with photons or protons (IMPT) may facilitate dose escalation within in