ESTRO 2023 - Abstract Book

S508

Sunday 14 May 2023

ESTRO 2023

Conclusion This large-scale data analysis, where all centres planned on the same dataset and planning goals, shows that subtle MLC errors can cause large dosimetric impact in spine SBRT cases. The of the impact varies between centres, the type of MLC is not a significant factor, and is clustered at the boundary between OAR and PTV. This indicates that the challenging trade- offs encountered in spine SBRT are sensitive to minor MLC errors which are well within machine tolerance. These findings indicate that the variations in impact are potentially due to plan optimisation approaches and highlight the need to carefully consider these delivery errors as part of plan robustness approaches. OC-0620 Simultaneously Integrated Protection in SBRT of Pancreas - An International Planning Benchmark Study C. Moustakis 1 , M. Grohmann 2 , D. Schmitt 3 , O. Blanck 4 , E. Gkika 5 , T. Brunner 6 1 University Hospital Muenster, Department of Radiation Oncology, Muenster, Germany; 2 University Medical Center Hamburg-Eppendorf, Department of Radiation Oncology, Hamburg, Germany; 3 University Medical Center Goettingen, Department of Radiation Oncology, Goettingen, Germany; 4 University Medical Center Schleswig Holstein, Kiel, Department of Radiation Oncology, Kiel, Germany; 5 University Hospital of Freiburg, Department of Radiation Oncology, Freiburg, Germany; 6 University Medical Center Graz, Department of Radiation Oncology, Graz, Austria Purpose or Objective The proximity or even overlap of PTVs and OARs pose a major challenge in stereotactic body radiotherapy (SBRT) of pancreatic cancer (PACA). The aim of this international planning benchmark study of the DEGRO/DGMP Working Group Stereotactic Radiotherapy was to investigate whether using Simultaneously Integrated Protection concept (SIP, [1]) in SBRT of PACA can lead to the improvement of planning quality. Materials and Methods 42 institutions from 5 countries participated in this study. Three particularly challenging cases with PTV and OAR overlap were selected from a community database after expert panel consensus. For each case, planning had to be performed with two multiparametric prescriptions in 5 and 8 fractions, respectively: a) D50%(GTV) = 5 x 9.2 Gy = 46 Gy (100%) and b) D50%(GTV) = 8 x 8.25 Gy = 66 Gy (100%) based on [2] while adhering to well-defined objectives and dose limitations for PTV/GTV and OARs. To investigate whether the SIP concept is used in current clinical practice and can lead to improved plan quality, the study was performed in two phases. In the first phase, necessary trade-offs for treatment planning were asked to be achieved according to routine in-house concepts. In the 2nd phase, re-planning was performed considering the SIP concept with specific provided contours from the expert panel including online training and study group discussion. Results A total of 292 plans were evaluated using a relative score based on [2] adapted to SIP planning by the expert panel. Most institutions could meet the GTV prescription requirements (Figure 1) and the majority already showed high plan quality using the SIP or similar concepts in the first phase of the study. Only 7 institutions did not use a SIP concept resulting in relatively lower plan quality scores. In the second phase, 3 of the 7 institutions were able to significantly improve their plan quality by using the SIP concept (Table 1). 18 of 19 institutions that participated in the second phase could at least maintain or improve their final score.