Comprehensive dose evaluation of a Deep Learning based synthetic Computed Tomography algorithm for pelvic Magnetic Resonance-only radiotherapy

Background and Purpose: Magnetic Resonance (MR)-only radiotherapy enables the use of MR without the uncertainty of MR-Computed Tomography (CT) registration. This requires a synthetic CT (sCT) for dose calculations, which can be facilitated by a novel Zero Echo Time (ZTE) sequence where bones are visible and images are acquired in 65 seconds. This study evaluated the dose calculation accuracy for pelvic sites of a ZTE-based Deep Learning sCT algorithm developed by GE Healthcare.Materials and Methods: ZTE and CT images were acquired in 56 pelvic radiotherapy patients in the radio-therapy position. A 2D U-net convolutional neural network was trained using pairs of deformably regis-tered CT and ZTE images from 36 patients. In the remaining 20 patients the dosimetric accuracy of the sCT was assessed using cylindrical dummy Planning Target Volumes (PTVs) positioned at four different cen-tral axial locations, as well as the clinical treatment plans (for prostate (n = 10), rectum (n = 4) and anus (n = 6) cancers). The sCT was rigidly and deformably registered, the plan recalculated and the doses com-pared using mean differences and gamma analysis.Results: Mean dose differences to the PTV D98% were <= 0.5% for all dummy PTVs and clinical plans (rigid registration). Mean gamma pass rates at 1%/1 mm were 98.0 +/- 0.4% (rigid) and 100.0 +/- 0.0% (deformable), 96.5 +/- 0.8% and 99.8 +/- 0.1%, and 95.4 +/- 0.6% and 99.4 +/- 0.4% for the clinical prostate, rectum and anus plans respectively. Conclusions: A ZTE-based sCT algorithm with high dose accuracy throughout the pelvis has been devel-oped. This suggests the algorithm is sufficiently accurate for MR-only radiotherapy for all pelvic sites.(c) 2023 The Author(s). Published by Elsevier B.V. Radiotherapy and Oncology 184 (2023) 1-7 This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).