ESTRO 2021 Abstract Book

S1671

ESTRO 2021

Conclusion Conclusion: In this cohort of patients observed initial analysis does not highlight any association between increased treatment delivery time and overall higher patient reported toxicity scores . Although rectal volumes fluctuated throughout again this study has found no correlation between rectal volume change and overall treatment delivery time . It is hoped further dosimetric evaluation, using DVH analysis, will further evidence that on treatment time is independent of associated treatment toxicity and aid in the essential,clinical on treatment decision making process of the RTT. PO-1964 Pre- and on-treatment rectal cancer target volume variations for tumor and involved lymph nodes. D.T. Arp 1,2 , A.L. Appelt 3,4 , M.S. Nielsen 1,2 , R. Mikalone 5 , L.Ø. Poulsen 6,2 1 Aalborg University Hospital, Department of Medical Physics, Department of Oncology, Aalborg, Denmark; 2 Aalborg University, Department of Clinical Medicine, Aalborg, Denmark; 3 University of Leeds, Leeds Institute of Medical Research at St James’s, Leeds, United Kingdom; 4 St James’s University Hospital, Leeds Cancer Centre, Leeds, United Kingdom; 5 Aalborg University Hospital, Department of Radiology, Aalborg, Denmark; 6 Aalborg University Hospital, Department of Oncology, Aalborg, Denmark Purpose or Objective Developments in rectal cancer treatment, e.g. increased use of IMRT and non-surgical management, have increased the demands on treatment delivery accuracy. Day-to-day variation in in tumour position has been studied, but corresponding data are needed for individual involved lymph nodes (GTV-N). This study examined variations in the position of the GTV-T and GTV-N on repeat MRI-scans before and during RT. Materials and Methods The study was based on interim data from an ongoing prospective clinical trial (AMPERE, NCT03619668) of repeat MRI scans before and during radiotherapy for rectal cancer. Patients are MRI scanned a total of six times; three before RT (on separate days) and three during RT (after one, two and four weeks of RT). T2 weighted sequences were performed in axial, sagittal and coronal direction on a 3T MRI scanner. GTV-T and one involved lymph node were delineated by an experienced consultant oncologist; with all delineations reviewed by a senior radiologist. Scans were co-registered using a rigid bony match to the first pre-treatment MRI (baseline). To characterize the variation in position of the GTV-T and GTV-N, the center-of-mass(COM) was calculated for each structure on each scan. The distance between COM on each MRI relative to the baseline was calculated for each structure. Furthermore, to examine the relative shift in position of GTV-N from GTV-T,