ESTRO 38 Abstract book

S1134 ESTRO 38

by the spacer distributor. Besides a CT examination, dedicated MR examinations were performed on a GE Discovery 750w. A T2-weighted image, covering the full body contour, was added to the clinical protocol and was used for sCT generation. A clinically approved VMAT treatment plan was created based on the CT and clinical local practice. For each patient, the generated sCT was resampled and rigidly registered to the CT. Structures were transferred to the sCT, including a structure delineating the spacer (see image for illustration). Separate body contours were generated based on the respective image. The CT based treatment plan was recalculated on the sCT, using the same number of MU. The dose distributions, based on sCT and CT, respectively, were compared based on the departments clinical DVH criteria. The mean HU value within the spacer structure was compared between the sCT and CT for all patients. Results The median dose differences [95% CI] between sCT and CT for D mean,PTV (%) and D mean,CTV (%) were 0.45% [0.35, 0.80] and 0.48% [0.40, 0.90], respectively. The median [95% CI] difference between sCT and CT for the rectum dose- volume constraint D 15% was 0.34% [0.26, 0.56]. Thus, the dose differences between sCT and CT plans for target structures and rectum were found to be small and not clinically significant. The mean HU within the spacer structure was -11.8±26.7 HU for sCT and 7.6±32.0 HU for CT. Conclusion The use of a hydrogel prostate rectum spacer is a straightforward procedure which fits well into our MRI only radiotherapy workflow. Minimal differences were found between sCT and CT dose distributions for both target and rectum, comparable to previous studies evaluating the technique without a spacer. The small differences in dose and HU indicate that the use of a prostate rectum spacer in an MRI only workflow is feasible with no clinically significant effect on the dose calculations.

Conclusion Simulations of low-dose paediatric CBCT scans show that a lower dose than is currently used clinically is sufficient for image evaluation in paediatric IGRT. As expected, a 50% reduction in dose leads to a 25% reduction in CNR. At 15% of clinically used doses CNR is reduced dramatically, but muscle and subcutaneous fat can still be distinguished. Investigation of the clinical impact of CNR reduction is planned. EP-2060 Feasibility of prostate rectum spacer in an MRI only radiotherapy workflow M. Lerner 1,2 , E. Persson 1,2 , N. Eliasson 1 , M. Moreau 1 , H. Benedek 1,3 , C. Gustafsson 1,2 , P. Nilsson 1 , E. Kjellén 1 , L.E. Olsson 1,2 , A. Gunnlaugsson 1 1 Skåne University Hospital, Hematology- Oncology and Radiation Physics, Lund, Sweden ; 2 Lund University, Translational Sciences- Medical Radiation Physics, Malmö, Sweden ; 3 Lund University, Medical Radiation Physics, Lund, Sweden Purpose or Objective A precondition for MRI only radiotherapy is the generation of a synthetic CT (sCT). Previous studies have successfully validated such methods for prostate radiotherapy. A prostate rectum spacer can reduce the dose to the main risk organs while maintaining high target dose. The aim of this clinical study was to validate the use of the spacer in an MRI only radiotherapy workflow by evaluating dose volume criteria and comparing Hounsfield Units (HU) within the spacer structure between the CT and sCT images. Since treatment planning is performed on the sCT, a validation of the spacer is required to ensure correct dose delivery. Material and Methods This study was approved by the Ethical Review Board and informed consent was obtained from all patients. Study participation did not affect treatment prescription, and routine clinical workflow was followed. Eight patients with localized intermediate risk prostate cancer were included. A hydrogel prostate rectum spacer was injected during gold fiducial marker insertion, by an oncologist certified

EP-2061 Feasibility of MR-only planning in a commercial treatment planning system H. McCallum 1 , N. Richmond 1 , C. Walker 1 , S. Andersson 2 , S. Svensson 2 1 The Newcastle Upon Tyne Hospitals NHS Foundation Trust, Northern Centre for Cancer Care, Newcastle upon Tyne, United Kingdom ; 2 RaySearch Laboratories, Research Department, Stockholm, Sweden Purpose or Objective MR is of increasing interest in radiotherapy treatment planning due to its superior soft-tissue contrast compared to CT. An MR-only pathway improves delineation accuracy and removes uncertainty due to CT-MR fusion. This study aims to investigate the feasibility of an MR-only planning solution which relies on bulk-density assignment which is fully integrated within the commercial treatment planning system RayStation.