ESTRO 2021 Abstract Book

S782

ESTRO 2021

were defined on T2-weighted three-dimensional turbo spin echo sequences, axial images (T2w 3D Tra) acquired on the MRL. To assess the guide’s utility, 8 RTTs with varying MRL experience contoured MR images from each of 5 fractions for 5 patients in the Monaco treatment planning system (Monaco 5.40.01), from 5 patients (n= 25 image data sets ). Each RTT contoured the prostate, seminal vesicles, bladder, and rectum on MR data sets before and after the introduction of the atlas. The ‘after’ contours were generated > 21 days following the ‘before’ contours to minimise the effects of repetition. Intra- and inter-observer contour variations (measured in volume), time to contour and observer contouring confidence were determined at both time-points using a 5-point Likert scale (1 being not confident and 5 being extremely confident). Results There were no statistically significant differences in any volumes (prostate or OARs) pre- and post- atlas introduction, with high correlations between all observers (R≥ 0.89) for all structures. Although the variance in volume definition decreased for all structures among all observers post intervention and the greatest in the prostate (mean variance before atlas 7.37 mm 3 to 3.47 mm 3 after) the change was not statistically significant. Contour overlap regions using Boolean structures are currently being calculated. Over all structures, the mean contouring time for all observers was reduced by 50%, from 53 to 27 minutes (range for no atlas 33 to 82 minutes and with atlas 22 to 55 minutes, p=0.01) following the introduction of the atlas (Figure 1). The mean contouring times for the prostate, bladder and rectum individually were reduced significantly from 14 to 7 minutes, (p=0.02) 14 to 7 minutes (p=0.002), 14 to 7 minutes (p=0.04) respectively. For all structures, the observer mean contouring confidence increased from 2.3 to 3.5 out of 5 (p≤0.02) (Figure 1).

Conclusion No statistically significant improvements were observed in contour variance amongst observers following the introduction of a consensus based atlas, the guidance suggesting decent baseline image interpretation. However, use of the atlas facilitated improved observer contour confidence and speed; key factors for a real- time RTT-led adapt-to-shape workflow. PD-0939 Feasibility of MR-guided tomotherapy on cranial & head-and-neck cases W.W. Fung 1 , S.Y. Man 1 , W.C. Leung 1 , K.F. Cheng 1 , O.L. Wong 2 , J. Yuan 2 , G. Chiu 1 , K.Y. Cheung 2 , S.K. Yu 2 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 To evaluate the use of setup room MRI for verifying cranial and head-and-neck (HN) tomotherapy under 1. Efficiency of the verification process; 2. Patient transfer stability and 3. Potential benefits of using MRI for Six patients (3 Cranium with 90frs & 3 HN with 100frs in total) were recruited as pilot study. For each fraction, patient was immobilized on a 1.5T MR simulator in a setup room. A 80s T1W sequence giving adequate anatomy information was scanned. The patient who remained in treatment position was then transferred to the opposite tomotherapy room by a patient transport board. MVCT thus treatment was performed without further setup. The time for each procedure was recorded. The position deviation between the MR (before transfer) & MVCT (after transfer) was assessed to evaluate the transfer stability. Daily MR to planning CT (pCT) shift i.e. MR shift was calculated and compared to the corresponding MVCT to pCT shift i.e. MVCT shift. This MVCT shift had already subtracted the patient transfer error to purely quantify the MR image quality effect on verification accuracy, which was considered comparable if the shift difference was <1mm/°. Paired-t test/Wilcoxon signed rank test was done. All matching assessments were performed using 6 DOF. treatment verification. Materials and Methods