An MR-only acquisition and artificial intelligence based image-processing protocol for photon and proton therapy using a low field MR

Lukas Zimmermann; Martin Buschmann; Harald Herrmann; Gerd Heilemann; Peter Kuess; Gregor Goldner; Tufve Nyholm; Dietmar Georg; Nicole Nesvacil

doi:10.1016/j.zemedi.2020.10.004

An MR-only acquisition and artificial intelligence based image-processing protocol for photon and proton therapy using a low field MR

Z Med Phys. 2021 Feb;31(1):78-88. doi: 10.1016/j.zemedi.2020.10.004. Epub 2021 Jan 15.

Authors

Lukas Zimmermann¹, Martin Buschmann², Harald Herrmann², Gerd Heilemann², Peter Kuess², Gregor Goldner², Tufve Nyholm³, Dietmar Georg², Nicole Nesvacil²

Affiliations

¹ Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria. Electronic address: lukas.fetty@meduniwien.ac.at.
² Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria.
³ Department of Radiation Sciences, Umeå University, Umeå, Sweden.

PMID: 33455822
DOI: 10.1016/j.zemedi.2020.10.004

Abstract

Objective: Recent developments on synthetically generated CTs (sCT), hybrid MRI linacs and MR-only simulations underlined the clinical feasibility and acceptance of MR guided radiation therapy. However, considering clinical application of open and low field MR with a limited field of view can result in truncation of the patient's anatomy which further affects the MR to sCT conversion. In this study an acquisition protocol and subsequent MR image stitching is proposed to overcome the limited field of view restriction of open MR scanners, for MR-only photon and proton therapy.

Material and methods: 12 prostate cancer patients scanned with an open 0.35T scanner were included. To obtain the full body contour an enhanced imaging protocol including two repeated scans after bilateral table movement was introduced. All required structures (patient contour, target and organ at risk) were delineated on a post-processed combined transversal image set (stitched MRI). The postprocessed MR was converted into a sCT by a pretrained neural network generator. Inversely planned photon and proton plans (VMAT and SFUD) were designed using the sCT and recalculated for rigidly and deformably registered CT images and compared based on D2%, D50%, V70Gy for organs at risk and based on D2%, D50%, D98% for the CTV and PTV. The stitched MRI and the untruncated MRI were compared to the CT, and the maximum surface distance was calculated. The sCT was evaluated with respect to delineation accuracy by comparing on stitched MRI and sCT using the DICE coefficient for femoral bones and the whole body.

Results: Maximum surface distance analysis revealed uncertainties in lateral direction of 1-3mm on average. DICE coefficient analysis confirms good performance of the sCT conversion, i.e. 92%, 93%, and 100% were obtained for femoral bone left and right and whole body. Dose comparison resulted in uncertainties below 1% between deformed CT and sCT and below 2% between rigidly registered CT and sCT in the CTV for photon and proton treatment plans.

Discussion: A newly developed acquisition protocol for open MR scanners and subsequent Sct generation revealed good acceptance for photon and proton therapy. Moreover, this protocol tackles the restriction of the limited FOVs and expands the capacities towards MR guided proton therapy with horizontal beam lines.

Keywords: MR-only simulation; Open MR scanner; Stitching protocol.

MeSH terms

Artificial Intelligence*
Humans
Image Processing, Computer-Assisted / methods*
Magnetic Resonance Imaging*
Photons / therapeutic use*
Proton Therapy*