ESTRO 36 Abstract Book

S490 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Treatment CT scans from up to 8 randomly selected patients included in the trial had target volumes auto delineated with MIM Maestro™ software version 6.5 (MIM Software Inc., Cleveland, OH) and manually edited according to ESTRO target volume delineation guidelines. Post editing of contours were verified by an observer (BVO),and considered as a gold standard reference (GS). Dice similarity coefficient (DSC) between original delineation (OD) and GS was calculated. Protocol compliance and dose distribution of delineated volumes (Dmin, Dmax, D2%, D95%, D98% and Homogeneity index (HI) for breast and nodal target volumes) were assessed in OD. HI and D95% were compared between OD and GS delineation of primary (CTVp),CTVn_L2-4- interpectoral (CTVn), internal mammary CTV (CTVn_ IMN) and CTVn_L1.

Conclusion This work presents a methodology to e stimate the parameters of a mechanistic, radiobiological l TCP model based on pre-treatment FMISO and FDG PET scans. The method is able to predict mean and median values of intra-treatment hypoxia for each of the lesions in a validation dataset held out from the analysis. This could potentially be used in the future to, for example, select patients for a de-escalation protocol based on their expected response. More patients will be added to the analysis in order to refine the prediction, find the defining characteristics of the outliers, and consolidate the results. PO-0891 Quality assessment of target volume delineation and dose planning in the Skagen Trial 1 G. Francolini 1 , M. Thomsen 2 , E. Yates 2 , C. Kirkove 3 , I. Jensen 4 , E. Blix 5 , C. Kamby 6 , M. Nielsen 7 , M. Krause 8 , M. Berg 9 , I. Mjaaland 10 , A. Schreiber 11 , U. Kasti 12 , K. Boye 13 , B. Offersen 14 1 Azienda Ospedaliera Universitaria Careggi, Department of Radiation oncology, Firenze, Italy 2 Aarhus University hospital, Department of Medical physics, Aarhus, Denmark 3 Catholic University of Louvain, Department of Radiation Oncology, Brussels, Belgium 4 Aalborg University Hospital, Department of Medical Physics, Aalborg, Denmark 5 University Hospital of North Norway, Department of Oncology, Tromso, Norway 6 Rigshospitalet, Department of Oncology, Copenhagen, Denmark 7 Odense University Hospital, Department of Oncology, Odense, Denmark 8 University Hospital Carl Gustav Carus, Department of Radiation Oncology, Dresden, Germany 9 Hospital of Vejle, Department of Physics, Vejle, Denmark 10 Stavanger University Hospital, Department of Oncology, Stavanger, Norway 11 Praxis für Strahlentherapie, Department of Radiation oncology, Dresden, Germany 12 Sørlandet Sykehus HF, Department of Oncology, Kristiansand, Norway 13 Zealand University Hospital, Department of Oncology, Naestved, Denmark 14 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Skagen Trial 1 is a multicenter, non-inferiority trial randomising early breast cancer patients to loco-regional irradiation with 50 Gy/25 fractions vs 40 Gy/15fractions. Primary endpoint is arm lymphedema. The protocol has pre-specified criteria for target volume delineation and dose planning, and quality assessment of this is reported. Inter-observer variability in delineation and its impact on dose parameters were assessed. Automated atlas-based segmentation was used in order to streamline assessment procedure.

Results 88 treatment plans from 12 centres in 4 countries were assessed. Delineation of all target volumes and organs at risk was complete in 99% and 96% of the patients,respectively. DSC showed high agreement in contouring, with average values of 0,9 for CTVp and 0,77 for nodal volumes.Complete dosimetric assessment was available in all patients for CTVp, but 1 patient with missing target volume delineation required integration with data extrapolated from GS. No deviations for target dose distribution were found in 76% of the patients, and 82% and 95%of the patients had successful target coverage of CTVp and CTVn, with 95% of volume covered by >95% and >90% of prescribed dose, respectively. Dose comparison for CTVp was performed in all patients, but 17 patients were excluded from CTVn comparison due to incomplete target delineation or exclusion of one or more nodal levels from target volume according to institutional policy. No differences were found for CTVp HI and D95% between OD and GS. CTVn, CTVn_L1 and CTVn_IMN were successfully covered (D95>90% of prescribed dose) in both delineations. Minimal differences were found in D95% and HI for CTVn (p<0,001 for both), CTVn_IMN (p=0,001 for D95%) and CTVn_L1 (p=0,02 for HI andp=0,03 for D95%).