ESTRO 2020 Abstract Book

S402

ESTRO 2020

using two-sample t -test. Interrater reliability was calculated by intraclass correlation coefficient (ICC) using ICC(3, k ) model defined by Shrout and Fleiss. Results The mean scores for TIQ and PIQ for the reference, P and CS image sets were reported in Table 1. TIQ and PIQ scores decreased monotonically as P or CS factors increased. The results suggested that acceleration factor up to 3 may be used without having unacceptable TIQ for the defined MM tasks as compared to the reference sequence, in which the frame rate could be increased by up to 2.8 times yielding higher temporal resolution. A severe degradation on TIQ and PIQ was observed when P or CS factors 5 was applied. In this qualitative IQ assessment, no statistically significant difference was found on the TIQ and PIQ scores between P and CS techniques ( p = 0.128) at 0.05 level of significance. ICC of TIQ scores for organ edge sharpness and anatomical details visibility were 0.76 (95% CI: 0.67- 0.84) and 0.84 (95% CI: 0.78-0.89) respectively, indicated a moderate to good interrater reliability. ICC of PIQ scores ranged from 0.38 to 0.92. Conclusion Using SENSE and Compressed SENSE acceleration techniques, improvement in shortening the dynamic scan time can be achieved for rapid morphological imaging with acceptable task-oriented image quality as compared to the vendor-default motion monitoring MR sequence in the MR- Linac. OC-0713 MR-linac boosts for patients with cervix cancer ineligible for brachytherapy; preliminary experience J. Hes 1 , A. Van Lier 1 , A. De Leeuw 1 , M. Den Hartogh 1 , P. Kroon 1 , E. De Groot-van Breugel 1 , I. Jürgenliemk-Schulz 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective In patients with cervix cancer elective external beam radiotherapy (EBRT) is preferably combined with brachytherapy (BT) boosts. In some cases BT is not feasible, due to anatomical reasons, comorbidity or other patient related factors. In these situations we currently combine elective EBRT with boosts on a conventional (CBCT) linac allowing for total target doses of about 70Gy. A wide PTV margin for the target is necessary to account for interfraction and intrafraction motion. The MR-linac (MRL) (Unity, Elekta AB) with its superior soft tissue contrast and possibility for online treatment planning according to the changing anatomy is a promising alternative. Here we present our first clinical experience with MRL boosts for the above mentioned situation. Material and Methods Recently 3 patients with cervical cancer, ineligible for BT were treated on the MRL. In our institute receiving 3 MRL boost fractions each. For each fraction relevant target and organ delineations have been adapted according to the daily anatomy as seen on MRI. IMRT-plans with 9 beams (7MV FFF) were generated using Monaco TPS (Elekta AB). The CTV to PTV margin was 3mm in all cases. Target dose above 70Gy EQD2 and OAR dose levels (as usually applied

Conclusion No severe acute GI or GU symptoms occurred during or four weeks after MR-RT for prostate cancer. Urinary frequency was the most severe GU symptom at the last treatment fraction. At the 4-week follow-up the severity of GU symptoms were considerably reduced. OC-0712 Improvement of respiratory motion monitoring using SENSE and Compressed SENSE for MR- guided RT Y. Ho 1 , O.L. Wong 1 , J. Yuan 1 , Y. Zhou 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Hong Kong, Hong Kong SAR China Purpose or Objective Orthogonal-cine MRI based on balanced steady-state free precession (b-SSFP) sequence is utilized in MR-guided radiotherapy as an organ motion monitoring (MM) tool. High temporal resolution would compromise image quality (IQ) in real-time MM. SENSE acceleration (P) and recently developed Compressed SENSE acceleration (CS) techniques allow fast imaging based on parallel imaging in image domain and data sparsity in k-space respectively. In this study, task-based IQ (TIQ) and perceived IQ (PIQ) were assessed to explore improvement of respiratory MM protocol using P and CS acceleration techniques. Material and Methods 9 sets of free breathing abdominal cine MRI with 300 dynamics were acquired for each of 5 healthy adult volunteers on a 1.5T MR-simulator in axial and coronal planes. Reference images were based on the vendor- default b-SSFP sequence in a 1.5 T MR-Linac without P or CS applied (TR/TE = 2.9/1.45 ms, FOV = 400 × 424 mm 2 , acquisition voxel size = 3 x 3 x 5 mm 3 , 1.2 fps). P images and CS images were acquired separately with acceleration factors 2 (2.2 fps), 3 (3.3 fps), 4 (4.2 fps) and 5 (5.1 fps) applied. All image sets were rated by 3 MRI specialists in a single-blinded randomised fashion using fixed-point scale (1 - 5: totally unacceptable – totally acceptable). TIQ scores were rated based on organ edge sharpness and anatomical details visibility for the task of motion monitoring. PIQ scores were rated based on IQ aspects including noise level, banding artefact, motion artefact and saturation artefact. P and CS scores were compared