ESTRO 2020 Abstract book

S237 ESTRO 2020

distortions related to the B 0 non-uniformity (system and pt related) scale linearly with the strength of the main magnetic field, and thus may be important to account for in high field MR-Linac treatments. The aim of this study is to estimate the tumour-site specific geometric distortion in pts treated for cancer in the adrenal gland, liver and prostate, respectively, on a 1.5 T MR-Linac. Material and Methods For this study 25 pts eligible for MR-Linac treatment were included prospectively (target was adrenal gland in 7 pts, liver metastasis in 3 pts and prostate in 15 pts). A 3D GRE dual-echo MRI sequence with echo times (TE) 4.6 and 9.2 ms (in-phase), was used to acquire phase and magnitude images on a 1.5 T MR-Linac (Unity, Elekta AB). B 0 maps were calculated with in-house developed software in MATLAB using the dual-echo method. The B 0 maps were converted to spatial distortion maps using the pixel bandwidth (693 Hz/pixel) and the pixel size (1.2 mm in readout direction) of a clinical 3D T2W SE sequence for the abdomen/pelvic region. Phase unwrapping was performed using a MATLAB function (sunwrap), and the axial through- target slice containing least unresolved phase wraps was used for distortion estimation. GNL related distortion (after 3D system correction) was calculated in 2D (axial plane) using the gradient reversal method and considered constant between pts. Results Representative target specific distortion maps related to B 0 (Figure 1a-c) and systematic GNL (Figure 1d) indicate large inter-patient variation. The mean distortions in the GTV are in the ranges 0.17 -1.92 mm (adrenal gland), 0.69 -1.55 mm (liver), and 0.06 - 1.86 mm (prostate) (Table 1), with no clear target dependent differences. Same levels of B 0 related distortion is seen in a 30 mm region around the GTV, indicating a spatial robustness of the estimates. The distortions related to the systematic GNL depend on the location of the tumour. For target sites within 100 mm from the iso-center, the distortion is below 1.0 mm, and for target sites within 150 mm the distortion can increase up to 1.4 mm. For target sites far off the iso-center (> 150 mm) the GNL may induce distortions up to 3.5 mm (Figure 1d). The largest GNL related distortion in presented dataset is 2.5 mm (pt 2, liver metastasis).

Conclusion The total geometric distortion is tumour site specific (scale with distance to iso-center) and can potentially reach clinically relevant levels in some pts. Ideally, the total distortion should be accounted for in a daily MRI-guided adaptive workflow, which particularly has the aim of delivering treatment with high geometric precision. PH-0408 Assessment of the generalizability to pediatric protontherapy of a 3D network generating pseudo-CT E. Alvarez Andres 1,2,3 , M. Caussé 2,3 , L. Fidon 1 , L. Ermeneux 2,4 , S. Bolle 5 , V. Martin 5 , N. Paragios 1 , E. Deutsch 3,5 , L. De Marzi 6,7 , C. Robert 2,3 1 TheraPanacea, Radiotherapy, Paris, France ; 2 Department of Medical Physics, Gustave Roussy - Paris- Saclay University, Villejuif, France ; 3 U1030 Molecular Radiotherapy, Paris-Sud University - Gustave Roussy - Inserm - Paris-Saclay University, Villejuif, France ; 4 U1018 Radiation Epidemiology, Paris-Sud University - Gustave Roussy - Inserm - Paris-Saclay University, Villejuif, France ; 5 Department of Radiotherapy, Gustave Roussy - Paris-Saclay University, Villejuif, France ; 6 Centre de protonthérapie d’Orsay, Institut Curie - PSL Research University, Orsay, France ; 7 U1021 Normal and Pathological Signaling: from the embryo to the innovative therapy of cancers, Institut Curie - Inserm - CNRS UMR 3347 - PSL Research University - Paris-Saclay University, Orsay, France Purpose or Objective A 3D convolutional neural network was used to map a Magnetic Resonance Imaging (MRI) into a pseudo Computed Tomography (pCT). It was trained and validated on an adults’ cohort and tested on a cohort including protontherapy-treated pediatric patients only to evaluate

the network robustness. Material and Methods

The total cohort was composed of 341 brain tumor patients leading to 162 pairs of CT/T1 weighted MRI (T1) and 179 pairs of CT/contrast-enhanced T1 weighted MRI (T1-Gd)