ESTRO 38 Abstract book

S97 ESTRO 38

The standard and the new RT setups are presented in Fig.1. Several comparative tests were performed.  Image quality test : 3D T1w TFE, 3D T2w TSE FLAIR and 2D T2w TSE scans were acquired for brain, for neck 2D T1w TSE and 2D T2w TSE mDIXON scans were acquired.

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SNR test : SNR maps were computed from a T1w scan (2 dynamics, the second being a noise scan) as in (Kellman P., et al. MRM (2005)54:1439-47). Motion restriction test: the maximum motion in the feet-head/left-right directions was estimated from 2D cine-MR T1w bFFE acquisitions (300 dynamics) using Optical Flow (Zachiu C., et al. Phys Med Biol (2015)60:9003– 29). Inter-fraction repositioning test : two high resolution 3D T1w sequences were acquired for each setup. Between acquisitions, the immobilization mask was removed and the subject was asked to move to mimic two different MRI sessions. The mean and the standard deviation of the subject movement between each pair of MR acquisitions were computed using Optical Flow as a proxy of the reproducibility of inter-fraction repositioning.

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Results Results are reported in Table 1. Three-dimensional errors were, on average, 1.34±0.19 mm and 1.54±0.35 mm for the ETS and the mask respectively. These uncertainties propagated into a maximum angular deviations of the fixation direction given to the patient by 0.6°, with an average of 0.26°.

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Conclusion The ETS position optimizer is capable of estimating the treatment setup on a geometrical level with errors lower than 1.5 mm. This translates into a deviation to the planned gazing angles below the threshold of clinical significance (1°). The proposed method automatically suggests the optimal treatment set-up geometry for a swifter treatment workflow and improved patient safety. OC-0189 Brain and Head-and-Neck MRI in immobilization masks: a novel and practical setup for radiotherapy S. Mandija 1 , F. D'Agata 1 , R. Navest 1 , A. Sbrizzi 1 , C. Raaymakers 1 , R. Tijssen 1 , M. Philippens 1 , E. Seravalli 1 , J. Verhoeff 1 , J. Lagendijk 1 , C. Van den Berg 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective In radiotherapy (RT), it is essential to perform the MRI and CT exams in treatment position. For this purpose and to minimize inter/intra-fraction movement, thermoplastic immobilization masks are used for brain/head and neck (HN) RT. However, standard immobilization masks are incompatible with diagnostic MR head/neck coils. As a compromise, flexible surface coils are adopted despite their technician dependent positioning and inferior signal- to-noise-ratio (SNR) compared to head/neck coils (2- channel vs. the 17-channel for head/neck receive coil). This leads to relatively poor image quality and reproducibility. Here, we explore the feasibility and performance of a new immobilization setup for brain/HN RT, redesigned to fit into the diagnostic head/neck MR coils thereby boosting MR image quality and reproducibility. Material and Methods MR images were acquired on 2 volunteers using a 3T Ingenia MRI (Philips Healthcare, Best, The Netherlands).

Results Brain images acquired with the standard RT coil setup had an inferior diagnostic quality compared to the new setup, especially for the FLAIR contrast (inferior sensitivity in detecting small lesions) (Fig. 2). For neck regions, the image quality was comparable between setups. The proposed setup allowed between 2 and 3 times higher SNR values for both brain/HN MRI (not shown – limited Fig. number). Comparable motion restriction in the feet-head/left-right directions (maximum motion ≈ 1 mm) and comparable repositioning accuracy (mean inter-fraction movement 1 mm ± 0.5 mm) were observed for the standard and the new setup.