ESTRO 2021 Abstract Book

S964

ESTRO 2021

contours of ITV and relevant OARs and a new IMRT plan was created. Results

Ten patients were treated and completed all of their planned 104 fractions on the MR-linac. The median duration of treatment was 41 minutes (range 32-70 minutes) with a median beam-on time of 8.9 minutes (Figure 1). No grade ≥3 acute toxicity was observed. In 2 patients, a significant decrease in ITV size of 58% and 37% was observed due to tumor shrinkage during treatment (Figure 2). In the other patients, the majority (81%) of online ITVs were within ±15% of the fraction 1 volume. Compared to the offline treatment plan, ITV coverage of the online plan was similar in 52%, better in 34% and worse in 14% of fractions. Reduced coverage was associated with preferentially meeting OAR constraints. Figure 1. Pie-chart demonstrating the different steps and respective average durations of the MR-guided online workflow.

Figure 2. Online MRI and dose distribution of the 1st (left) and 12th (right) fraction of a patient with a mediastinal lymph node metastasis of a SCLC in whom a 58% decrease in ITV was observed during treatment. The ITV (green) and OARs (esophagus (red), trachea (blue), spinal cord (purple)) within a 3 cm ring (yellow) around the PTV (green) are modified during treatment by a radiation oncologist.

Conclusion We demonstrate the safety and feasibility of SBRT of (ultra)central lung tumors on an 1.5 T MR-Linac. All patients completed all treatment fractions on the MR-linac with improved ITV coverage in 34% of fractions through daily plan adaptation. In 2 patients, a significant decrease in ITV size was observed during treatment, illustrating the additional benefits of MRI-guided radiotherapy. PO-1162 DLCA-R: Variation in elderly stage III NSCLC treated with definitive chemoradiation. E. Dieleman 1 , L. van der Woude 2,3 , E. Vonk 4 , L. van Bockel 5 , I. Coremans 6 , C. van Es 7 , K. de Jaeger 8 , H.P. Knol 9 , W. Kolff 1 , F. Koppe 10 , J. Pomp 11 , B. Reymen 12 , D. Schinagl 13 , F. Spoelstra 14 , C. Tissing-Tan 15 , F. Ubbels 16 , N. van der Voort van Zyp 17 , A. van der Wel 18 , R. Wijsman 16 , M. Dielwart 19 , E. Wiegman 20 , J. Belderbos 21 1 Amsterdam UMC location AMC, Radiation Oncology, Amsterdam, The Netherlands; 2 RadboudUMC, Cardiothoracic surgery, Nijmegen, The Netherlands; 3 Dutch Institute for Clinical Auditing,, -, Leiden, The Netherlands; 4 Radiotherapie groep, Radiation Oncology, Deventer, The Netherlands; 5 Haga ziekenhuis, Radiation Oncology, Den Haag, The Netherlands; 6 LUMC, Radition Oncology, Leiden, The Netherlands; 7 UMCU, Radiation Oncology, Utrecht, The Netherlands; 8 Catharina Ziekenhuis, Radiation Oncology, Eindhoven, The Netherlands; 9 Noordwest Ziekenhuis groep, Radiation Oncology, Alkmaar, The Netherlands; 10 Instituut