Validation and clinical implementation of an MRI-only prostate cancer radiotherapy workflow

Abstract: The radiotherapy workflow for prostate cancer is associated with systematic uncertainty stemming from the registration between the computed tomography (CT) and magnetic resonance (MR) images. A radiotherapy workflow based solely on MR imaging (MRI), called an MRI-only workflow, has been suggested as a means of eliminating this uncer¬tainty. The aim of the work presented in this thesis was to validate and clinically implement an MRI-only workflow for prostate cancer.Several aspects of the implementation of an MRI-only workflow have been investigated in the work presented in this thesis. In the registration process between MR and CT images using fiducial markers, the observer bias was found to displace the estimated target position by up to 3 mm, compared to the clinical baseline. The delineated prostate volume was, on average, 18% smaller in the MRI-only delineation procedure than in dual-modality delineation. If this difference is not accounted for, a reduction in the treated volume could arise in the implementation of an MRI-only workflow. Both registration and target delineation uncertainties manifest as systematic deviations for each patient in the dual-modality workflow, which are elimi¬nated in an MRI-only workflow. MRI-only treatment planning employing the synthetic CT (sCT)-generation software MriPlannerTM, was validated in a multi-centre/multi-vendor study. The method was found to be robust for a variety of MRI vendors, magnetic field strengths, prescriptions and treatment planning strategies. In the spring of 2017, the first MRI-only-based treatment in Sweden using this software was delivered in a clinical study in Lund. A total of 39 patients were treated in this study using a prospective implementation approach together with an MRI-only workflow. Using a new single-sequence strategy, image registration between different image volumes was eliminated. One patient was excluded due to obesity. CT imaging was included in the workflow for quality assurance (QA) purposes. Acceptance criteria for dose calculations were confirmed within a 2% dose deviation and 98% gamma pass rate. Fiducial marker identification was successfully performed with 100% detection accuracy using MR images. Patient set-up verification was performed, and was within 2 mm of the CT-based set-up verification for most patients. In the clinical use of an MRI-only workflow there will be no need for CT imaging. An sCT QA method using cone beam CT (CBCT) images was developed to completely remove the need for CT imaging in a clinical MRI-only workflow. CBCT images successfully replaced the CT images in the suggested QA method for sCT images. In conclusion, the work presented in this thesis demonstrates that an MRI-only workflow for radiotherapy of prostate cancer can be clinically implemented.