ESTRO 36 Abstract Book

S83 ESTRO 36 _______________________________________________________________________________________________

About 80% of absolute shift errors in all MR-P xy s were not significantly different from MR-ref in all directions. No specific trend between PAT-f and shift error was observed, implying fusion accuracy could not be predicted by PAT-f value. The errors of T2W were significantly smaller than T1W in almost all of the shifts (p<0.05). Junior observers performed significantly better than seniors when fusing T1W images (p<0.05). For treatment verification purpose, T1W MR-P 42 (88s) and T2W MR-P 22 (173s) were chosen since they were the fastest sequences with comparable image quality to MR-ref. However when considering fusion errors, MR-P 44 for T1W (49s) and T2W (59s) images, with an even shorter scan time, should be opted instead as its verification accuracy had no significant difference in most shift directions from MR-ref (Fig.2) despite significantly degraded image quality.

Conclusion Despite standardized information and performing simulation on the treatment machine, anxiety remains an item that needs specific attention. Even with fraction duration times of up to 75 minutes, only a single patient perceived this as being unacceptably long. OC-0162 Optimizing sequences for MRI-guided radiotherapy in cranial and head and neck regions W.W.K. Fung 1 , S.Y. Man 1 , J. Yuan 2 , L.H. FUNG 2 , W.P. LUK 2 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Happy Valley, Hong Kong SAR China 2 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective MR sequences using parallel acquisition technique (PAT) with increasing acceleration factors could reduce the scan time for treatment verification, but with the cost of losing image quality that could affect verification accuracy. This study assessed the effect of different PAT factors on image quality, scan time and fusion accuracy, thus choosing a sequence which is clinically suitable for MRI-guided RT in cranial (C) and HN regions. Material and Methods Ten healthy volunteers were set up in treatment position using headrest and immobilization mask on the flat couch of a 1.5T MRI-simulator (Siemens MAGNETOM Aera). High resolution isotropic (1.05mm) 3D TSE T1W and T2W MR sequences were acquired (MR-ref). Based on MR-ref, 11 low-resolution (isotropic 1.4mm) verification sequences (MR-P xy s) were acquired with GRAPPA where acceleration factors x and y were altered in respectively phase encoding and slice encoding directions. Effective PAT factor (PAT-f) equals x times y. Four therapists (2 seniors & 2 juniors) performed two sets of fusions: MR-ref & MR- P xy s and MR-ref & duplicated MR-ref (control set) for C and HN region. Shift results (6DOF) were recorded. Survey was given to observers for scoring the image quality. Logistic and linear regression were used. Results The scan time for MR-ref were 301s and 330s, and for MR- P xy s it ranged from 249s to 49s and 254s to 59s for T1W and T2W images respectively when PAT-f increased from 3 (MR-P 31 ) to 16 (MR-P 44 ). Subjective analysis showed that the scores of all verification series were lower than the reference and decreased with increasing PAT-f. Image quality decreased when reducing the scan time (Fig.1). Significant reduction of image quality (p<0.05) occurred when PAT-f reached 12 for T1W and 6 for T2W images. Observers favored T1W over T2W images (p<0.0001). Scores from senior observers were significantly better than juniors (p<0.0001).

Conclusion The degradation of image quality by increasing PAT-f does not necessarily affect the fusion accuracy. MR-P 44 with acceptable verification accuracy and shortest scan time is therefore proposed for MRI-guided RT in C and HN regions. OC-0163 Online workflow for the First-in-Man study on bone metastases at the MRI-linear accelerator L.T.C. Meijers 1 , S.J. Hoogcarspel 1 , A.N.T.J. Kotte 1 , C.N.N. Nomden 1 , G.G. Sikkes 1 , I.H. Kiekebosch 1 , E.N. Groot de 1 , G.H. Bol 1 , B. Asselen van 1 , I.M. Jurgenliemk- Schulz 1 , L.G.W. Kerkmeijer 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiotherapy department, Utrecht, The Netherlands Purpose or Objective The first treatment on the MR-Linac in the UMC Utrecht includes patients with bone metastases, the first-in-man study. A fast workflow is required since all processes will be performed in an online setting. The aim of the present study is to test the feasibility of an online workflow for these patients at the MR-Linac within a 30 minute time limit. Material and Methods A workflow with offline and online procedures was developed for the MR-Linac. To test this workflow, the Alderson phantom was used, a human body shaped phantom with tissue-equivalent material. The phantom yields sufficient CT and MR contrast. The offline workflow included acquisition of a reference CT scan and manual delineation of the spinal cord and one lumbar vertebra (vertebra = CTV, PTV-CTV margin = 5 mm). The online workflow at the MR-Linac, shown in figure 1, included setup of the patient (= Alderson phantom) and acquisition