ESTRO 38 Abstract book

S1124 ESTRO 38

Engineering, Gothenburg, Sweden ; 3 University of Gothenburg, Department of Oncology and Radiotherapy- Institute of Clinical Sciences- Sahlgrenska Academy, Gothenburg, Sweden Purpose or Objective The MR only radiotherapy workflow is based solely on MR data, hence there is no CT data available. Instead a synthetic CT (sCT) generated from the MR data is used as Hounsfield Unit (HU) map for dose calculation. Consequently, the Digital Reconstructed Radiograph (DRR) originating from CT data is replaced with synthetic DRR (sDRR) (Figure 1). The purpose of this observer study was to investigate the precision of patient positioning for head and neck cancer using sDRR, compared to the today clinical used DRR.

Conclusion Differences in all degrees of freedom for sDRR-DRR are found to be similar to the differences for defDRR-DRR, i.e. the generation of sCT is not introducing any large additional uncertainties or errors that propagates to the sDRR. The similarity indicates that the dominating factor contributing to the observed differences is repositioning between the CT and MR scan. Supplementary phantom measurements, enabling fixed coordinate systems, could be valuable. EP-2047 Investigating a new MR sequence combined with radiologist training for prostate delineation J. Wyatt 1 , J. Frew 1 , A. Henry 2,3 , L. Murray 2,3 , R. Pearson 1 , A. McNeil 4 , E. Johnstone 2 , R. Speight 3 , H. McCallum 1 1 Newcastle upon Tyne Hospitals NHS Foundation Trust, Northern Centre for Cancer Care, Newcastle upon Tyne, United Kingdom ; 2 University of Leeds, Leeds Institute of Cancer and Pathology, Leeds, United Kingdom ; 3 Leeds Teaching Hospitals NHS Trust, Leeds Cancer Centre, Leeds, United Kingdom ; 4 Newcastle upon Tyne Hospitals NHS Foundation Trust, Radiology, Newcastle upon Tyne, United Kingdom Purpose or Objective MR is increasingly being used for prostate delineation in radiotherapy due to its superior soft-tissue contrast. However the literature is scarce regarding the optimal sequence for prostate definition and many oncologists are unfamiliar with using MR images for delineation. This study aimed to investigate the variability of prostate delineation for two different MR acquisition sequences both prior to and after expert radiologist training. Material and Methods CT and MRI scans for radiotherapy planning were acquired in the treatment planning position for 15 patients in two cohorts (5 and 10 respectively). Two MR sequences were acquired in the same scanning session, a 3D T2-weighted turbo spin echo sequence (SPACE) and a 2D T2-weighted combined multiple gradient echo sequence (MEDIC). Three consultant oncologists from two institutions delineated the prostate and seminal vesicles on each image set independently for the first cohort using the same treatment planning system. These delineations were then

Material and Methods Pre-treatment CT and MR from four patients, as well as kV-images (0⁰ and 90⁰) from the linac mounted imaging device were used. sCT data was generated using MriPlanner (Spectronic Medical AB). A second CT data set was created by deformable registration of the CT to the MR using Velocity (Varian Medical Systems). The deformed CT data was used to mitigate anatomical differences between the pre-treatment imaging sessions. For each patient, orthogonal kV images from 4 fractions were retrospectively manually registered in five degrees of freedom against 3 different image sets (DRR, sDRR and deformed DRR (defDRR)) by 6 observers, resulting in a total of 288 registrations. None of the observers were informed about which type of DRR they worked with. Results Differences between patient positioning with sDRR and the clinically used DRR, as well as differences between defDRR and DRR, in vertical (vrt), longitudinal (lng) and lateral (lat) directions, for all observers, are presented in Figure 2. The mean differences and the standard deviations between patient positioning with sDRR and DRR were - 0.79±1.75 mm, -0.59±0.79 mm and 0.96±1.20 mm in vrt, lng and lat direction respectively. For pitch and rotation (rtn), a difference of -0.61±1.41⁰ [-4.0 2.6] and - 0.22±0.83⁰ [-2.7 1.7] where noticed. The differences between defDRR and DRR concluded in a mean difference of -1.08±2.36 mm, -0.53±1.08 mm and 0.53±1.26 mm in vrt, lng and lat directions and in pitch and rtn of - 0.68±1.33 [-3.8 2.4] and -0.31±0.87⁰ [-3.5 1.6].