Metal artifact reduction for 3 T MRI-only prostate radiotherapy in patients with hip prostheses: impact on target definition, synthetic CT generation, dose calculation, and fiducial marker identification

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BACKGROUND: Magnetic resonance imaging (MRI) offers superior soft-tissue contrast and MRI-only radiotherapy (RT) eliminates uncertainties associated with computed tomography (CT)-to-MRI image registration. An MRI-only RT workflow in patients with hip prostheses remains challenging since delineation, synthetic CT (sCT) generation, and marker identification ideally rely on MR images, which are sensitive to distortions. Metal-artifact reduction sequences (MARS) mitigate metal distortions but often obscure gold markers. The aim of this study is to present a proof-of-concept 3 T MRI-only RT workflow for prostate cancer patients with hip prostheses, using only MARS MAVRIC-SL (GE HealthCare) for delineation, sCT generation and fiducial marker identification. METHODS: Five prostate cancer patients with one or two hip prostheses and intraprostatic fiducial markers underwent CT, standard 3 T MRI and additional MAVRIC-SL MRI. All images were used retrospectively and did not affect patient treatment. Delineation and sCT generation were performed using MAVRIC-SL. Intraprostatic markers were not visible in MAVRIC-SL due to artifact suppression; instead, raw data were reconstructed into spectral-bin images to manually identify the markers. To prevent incoming radiation from passing through the implants and reduce the associated uncertainties in dose calculation and treatment delivery, beam avoidance sectors (AS) were applied. Conventional CT-based avoidance sectors were registered to the sCT and defined as AS1. Prosthesis-induced signal voids in MAVRIC-SL were used to define AS2. Two treatment plans (42.7 Gy, 7 fractions) were created per patient using AS1 and AS2. Plans were compared using dose-volume histogram (DVH) parameters. AS2-plans were recalculated on registered CTs for DVH-comparison and gamma analyses (2%/2 mm and 1%/1 mm with 10% dose threshold) were performed. RESULTS: Delineation, marker identification and sCT generation were successful using MAVRIC-SL. Median dose differences between AS1 and AS2 remained within ± 0.1% for PTV, while OAR metrics ranged from - 0.4% to + 0.7%. AS2-plans recalculated on CT showed dose differences ≤ 0.9% for PTV and ≤ 1% for OARs. Median gamma pass rates were 99.1% (2%/2 mm) and 96.4% (1%/1 mm). CONCLUSIONS: This study demonstrates the feasibility of an MRI-only radiotherapy workflow at 3 T for prostate cancer patients with hip prostheses, enabling delineation, sCT generation, dose calculation and marker identification from a single MAVRIC-SL acquisition. CLINICAL TRIAL NUMBER: Not applicable.

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