ESTRO 2020 Abstract book

S959 ESTRO 2020

relatively stable in the rectum for a 20 minute period. Changes are more likely to occur at the cranial aspect of the rectum than the caudal end. This work will feed into assessment of the total dosimetrical effect of rectal gas during MRgRT. However, in practical MRgRT the relevant time period may exceed our measurement period. PO-1652 In silico trial of cone-beam-CT guided adaptive planning for radiotherapy of locally advanced NSCLC P. Hoegen 1 , C. Lang 2 , S. Akbaba 1 , P. Häring 2 , M. Splinter 2 , A. Miltner 2 , M. Bachmann 2 , C. Stahl-Arnsberger 2 , T. Brechter 2 , D. Bernhardt 1 , S. Klüter 1 , M. Syed 1 , F. Weykamp 1 , L. König 1 , J. Debus 1 , J. Hörner-Rieber 1 1 University of Heidelberg, Department of Radiation Oncology, Heidelberg, Germany ; 2 German Cancer Research Center DKFZ, Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany Purpose or Objective To evaluate the potential of cone-beam-CT (CB-CT) guided adaptive radiotherapy (ART) for locally advanced non- small cell lung cancer (NSCLC) in terms of sparing of surrounding organs-at-risk (OARS). Material and Methods In 10 patients with locally advanced NSCLC, daily CB-CT imaging was acquired during radio- (n=3) or radiochemotherapy (n=7) for simulation of ART. Patients were treated with conventionally fractionated intensity- modulated radiotherapy (IMRT) with total doses of 60-66 Gy (pPlan) (311 fraction CB-CTs). OARs were segmented on every daily CB-CT and the tumor volumes were modified weekly depending on tumor changes. Doses actually delivered were recalculated on daily images (dPlan), and voxel-wise dose accumulation was performed using a deformable registration algorithm. For simulation of adaptive radiotherapy (ART), treatment plans were adapted using the new contours and re-optimized weekly CB-CT showed continuous tumor regression of 1.1 % ± 0.4 % per day, leading to a residual gross tumor volume (GTV) of 65.3 % ± 13.4 % after 6 weeks of radiotherapy (p=0.005). Corresponding PTVs decreased to 83.7 % ± 7.8 % (p=0.005). In the actually delivered plans (dPlan), both conformity (p=0.005) and homegenity (p=0.06) indices were impaired compared to the initial plans (pPlan). This resulted in higher actual lung doses than planned: V20 was 34.6 Gy ± 6.8 Gy instead of 32.8 Gy ± 4.9 Gy (p=0.01), mean lung dose was 19.0 Gy ± 3.1 Gy instead of 17.9 Gy ± 2.5 Gy (p=0.07). Weekly plan adaptation enabled both decreased lung V20 of 31.6 Gy ± 6.2 Gy (p=0.007) and mean lung dose of 17.7 Gy ± 2.9 Gy (p=0.005). Target volume coverage represented by conformity and homogeneity indices could be improved by weekly plan adaptation (CI: p=0.007, HI: p=0.114) and reached levels of the initial plan (CI: p=0.72, IGRT with CB-CT detects continuous GTV and PTV changes. CB-CT-guided adaptive radiotherapy for locally advanced NSCLC is feasible and allows for superior sparing of healthy lung at high levels of plan conformity. PO-1653 Dosimetric effects of interfractional variations in prostate cancer radiotherapy T. Bostel 1,2 , I. Sachpazidis 3 , M. Splinter 2,4 , N. Bougatf 2,5 , T. Fechter 3 , C. Zamboglou 3 , O. Jäkel 4 , P. Huber 2,5 , D. Baltas 3 , J. Debus 2,5 , N. Nicolay 2,3 1 Mainz University Hospital, Radiation Oncology, Mainz, Germany ; 2 German Cancer Research Center DKFZ, Clinical Cooperation Unit Radiation Oncology, Heidelberg, Germany ; 3 Medical Center – University of Freiburg, Radiation Oncology, Freiburg, Germany ; 4 German Cancer Research Center DKFZ, Medical Physics (aPlan). Results HI: p=0.33). Conclusion

in Radiation Oncology, Heidelberg, Germany ; 5 Heidelberg University Hospital, Radiation Oncology, Heidelberg, Germany Purpose or Objective This prospective study aimed to analyze deviations of the applied radiation doses from the radiation plan for definitive and postoperative radiotherapy of prostate cancer as a function of interfractional anatomical variations and image frequency. Material and Methods In 20 patients who received definitive or postoperative intensity-modulated radiotherapy of the prostate or prostatic fossa (10 patients in each group), daily diagnostic in-room CT scans were performed in treatment position for position verification (680 fraction CTs). Target volumes and organs-at-risk were contoured on each scan by a board-certified radiation oncologist. Applied fraction doses were recalculated on the daily images and mapped to the planning CT. Voxel-wise dose accumulation was performed using a deformable registration algorithm. To simulate weekly imaging, weekly position correction vectors were used from the first scan of each treatment week for each patient to rigidly register all daily scans of the respective treatment week onto the planning CT before dose accumulation. Detailed dose statistics of the prescribed and applied treatment doses were compared in relation to the frequency of position verification imaging. Derived tumor control (TCP) and normal tissue complication (NTCP) probabilities were calculated. Results Despite a large variability in the pelvic anatomy, daily CT- based patient repositioning resulted in largely negligible deviations of the analyzed dose-volume, conformity and uniformity parameters from the planned doses for both definitive and post-prostatectomy radiotherapy, with the exception of the urinary bladder which exhibited significant increases in the accumulated mean and median doses in the postoperative setting. Average TCP values were comparable for definitive treatment, and the NTCP values for the applied doses to the bladder and rectum did not significantly deviate from the treatment plan for both definitive and postoperative treatment. In contrast, weekly CT-based repositioning in definitive prostate cancer radiotherapy resulted in significant decreases of the PTV coverage and dose conformity as well as large deviations of the applied doses to the rectum and bladder from the planned doses. In the post-prostatectomy group, the coverage and conformity of the applied doses to the CTV and PTV significantly deviated from the planned doses, and the calculated NTCP values for the bladder and rectum were found erroneously reduced for weekly patient repositioning. Conclusion Our data indicate for the first time in a voxel-by-voxel analysis that daily imaging is required for reliable adaptive delivery of intensity-modulated radiotherapy to the prostate and prostatic fossa. This work will contribute to devising adaptive re-planning strategies for both prostate and post-prostatectomy radiotherapy respectively. PO-1654 Markerless liver stereotactic body radiotherapy on a 1.5 T MR-Linac C. Gani 1 , B. Simon 1 , M. Nachbar 2 , A. Stolte 1 , J. Boldt 1 , C. Marks 1 , D. Thorwarth 2 , D. Zips 1 1 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany ; 2 University Hospital Tübingen, Department of Radiation Oncology- Section for biomedical physics, Tübingen, Germany Purpose or Objective Stereotactic body radiotherapy (SBRT) is a highly effective method for the treatment of both primary liver tumors and liver metastasis. However with cone-beam computed tomography, only few tumors can be visualized