ESTRO 37 Abstract book

S1134

ESTRO 37

Conclusion This audit demonstrates that ~40% of adaptive assessments on CBCTs are for lung patients with anatomical changes. Despite this only ~30% of all replans are lungs. A similar situation is true for H&N plans which take up 32% of CBCT adaptive assessments but only result in 18% of replans. Our replan rate of 1.9% agrees a previous study 1 , stating that <5% of patients are likely to need a replan. This work will aid the development of improved IGRT protocols reducing the number of plans that are referred for adaptive assessment thus improving the treatment workflow and allowing resources to be directed where they are most needed. It will also inform the workflow design of new technology such as PBT and the MR-Linac. Reference 1 Rowbottom C, The Practical “costs” of adaptive radiotherapy, ESTRO 35, 2016, SP-0394 EP-2064 Intra- and inter-fractional motion in radiotherapy of rectal cancer quantified using MRI and CBCT C. Skinnerup Byskov 1 , L. Nyvang 1 , A. Harbøll 2 , A. Schouboe 2 , E.M. Pedersen 3 , C.J.S. Kronborg 2 , O. Casares- Magaz 1 , P.R. Poulsen 1 , K.L.G. Spindler 2,4 , L.P. Muren 1 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark 2 Aarhus University Hospital, Oncology, Aarhus C, Denmark 3 Aarhus University Hospital, Radiology, Aarhus C, Denmark 4 Aarhus University Hospital, Experimental Clinical Oncology, Aarhus C, Denmark Purpose or Objective Adaptive radiotherapy (ART) has a potential to account for the complex patterns of internal pelvic motion that occur during the course of treatment of rectal cancer. However, the cone-beam CT (CBCT) scans available for online image-guidance and adaptation have limited soft- tissue contrast, in particular in the pelvic region. The aim of this study was therefore to investigate the ability of CBCT to assess inter- and intra-fractional motion patterns of the rectum by comparing motion patterns extracted from repeat CBCTs with those of magnetic resonance imaging (MRI). Material and Methods The rectum was delineated on weekly CBCT scans (N = 71 scans) of 12 patients with rectal adenocarcinoma treated with neo-adjuvant radiotherapy (50.4 Gy, 28 fractions). For comparison of image modalities, three of the patients had additional weekly repeat MRIs and post-treatment CBCTs acquired, where the rectum was delineated as well. Using the rectum delineations from all modalities, 2D maps of the distance between the delineated rectum surface and the rectum centroid in each slice were created by virtually unfolding all contour slices. The radial difference between contours was then measured as the difference of this distance for corresponding contour points (Fig. 1). Intra-fractional motion patterns were investigated as the contour differences in time-resolved MRIs (6 x cine, T2W, voxel size: 0.78x0.78x4.4 mm, scan time: 41 s (total: 9 min)) and between pre- and post- treatment CBCTs (median time difference: 6 minutes); inter-fractional motion patterns were described by the differences in the weekly repeat MRIs and CBCTs.

Results The rectum contour differences were in general much larger between inter-fractional scans compared to intra- fractional scans. The percentage of surface contour points more than ± 5 mm away from the planning CT (pCT) contour was 16% (median; range: 0-64 %) for CBCT inter and 12% (median; range: 0-29%) for MR inter whereas for CBCT intra and MR intra the percentages were 0% (median; range: 0-2%) and 0% (median; range: 0-7 %). Delineations on MRI and CBCT scans all showed differences of similar magnitude. Rectal volumes delineated on CBCTs were smaller than the corresponding pCT rectal volumes in 86% of the cases, which resulted in a large proportion of contour points lying more than 5 mm within the pCT contour (Fig. 2).

Conclusion Pelvic CBCT scans were of sufficient quality to analyse inter-and intra-fractional rectal motion with similar motion patterns as in MRI scans. Inter-fractional motion was considerably larger than intra-fractional motion. Changes in the rectal volume on CBCT (compared to the planning scan) were associated with a large proportion of surface contour points being more than 5 mm away from the pCT contour. EP-2065 Is there a need for adaptive radiotherapy for head and neck- cancer patients on a routinely basis? K. Sund Solli 1 , H. Tøndel 1 , S.S. Almberg 1 1 St. Olavs University Hospital Trondheim, Cancer Clinic, Trondheim, Norway Purpose or Objective To evaluate the robustness of Volumetric-Modulated Arc Therapy (VMAT) for head and neck (H&N) cancer treatment. The patients frequently experience weight loss and tumor shrinkage during treatment, which in turn may affect both the anatomy and the dose distribution in the patient. Material and Methods Between February 2016 and October 2016, twenty consecutive patients with locally advanced head and neck cancer were included in this study. All patients were treated with VMAT and simultaneously integrated boost. Target dose was at least 60 Gy (range: 60-68 Gy) and all patients received elective nodal irradiation to 54 Gy. CTV-PTV margin of 5 mm was applied. After 3 weeks of