About this Research Topic
Since the introduction of magnetic resonance imaging (MRI) to radiation therapy (RT), it has been increasingly adopted in RT treatment planning for target and organ-at-risk (OAR) definition due to superior soft tissue contrasts. Recently, roles of MRI in the field of radiation oncology have been extended to MRI-based simulation, motion management, RT guidance, and geometric and physiologic-based adaptation. These advancements are due to the development of dedicated MR simulators, integration of MR scanner with radiation treatment platform, as well as technical developments and applications of tumor tracking, adaptive planning, and treatment response evaluation.
Although MRI has been used in RT treatment planning for decades, dedicated MR simulators (MR-Sim), introduced in recent years, start to consider unique RT requirements and workflows. These efforts include modifying RF coils to couple with RT immobilization devices, providing flat table tops, external laser systems and isocenter marking software for patient positioning, optimizing MR sequences for RT planning and organ motion management, and automated MR-synthesized CT for dose calculation. These technical developments and integrations show great promise when using MRI as the primary imaging modality for RT treatment planning and simulation, which has the potential to improve treatment outcomes and reduce toxicity.
Furthermore, recent commercial developments of MRI-guided RT platforms (MRgRT) provide great opportunities for direct imaging guidance, tumor/OAR tracking during RT, and treatment adaptation. Two systems have received CE and/or FDA 510k clearance so far: the Unity MR-Linac (Elekta, Stockholm, Sweden) combines a modified 6MV Elekta (Stockholm, Sweden) Linac and 1.5T Philips (Best, Netherlands) Ingenia MRI system. Both of these monitor and track real-time tumor motion, allow for the targeting of tumors with higher doses without an increase in OAR doses, which has the potential to improve tumor control. The advantages of MRgRT have just started to be realized.
Multiparametric MRI (a combination of morphologic and functional imaging modalities) has shown the potential to increase the accuracy of tumor detection, localization, and characterization of cancer aggression. Diffusion weighed MRI (DWI) measure the Brownian motion of water molecules in tissue and reflects the cell density of tissues. Dynamic contrast-enhanced (DCE) MRI assesses tumor angiogenesis and detects microvascular vessel wall permeability. Blood oxygen level dependent (BOLD) MRI provides metabolic and oxygenation information pertaining to local microenvironmental properties in the tumor. MR spectroscopy (MRS) can measure metabolite compositions in cancerous tissue, which has shown promising results in tumor delineation for prostate cancer and glioblastoma. Combining multiparametric MRI techniques with MRI simulation and guidance in RT offers enormous opportunities to individualize RT adaptation based upon individual patient’s response to treatment.
The goal of this Research Topic is to cover a broad range of the advancements of MRI in RT and to evaluate the clinical impacts of MRgRT on tumor control and clinical outcomes.
This collection welcomes the following themes:
• Target volume delineation and margin determination using MRI as the primary imaging modality
• Development of integrated coils and immobilization devices for MR-Sim and MR-Linac
• Geometrical accuracy and spatial/temporal resolutions of MR sequences used for RT
• Techniques to generate synthetic CT from MRI
• 4D MRI acquisition and reconstruction techniques for organ motion management
• Techniques for tumor and ORA motion tracking on MR-Linac
• On-line adaptive treatment planning strategies
• Quantitative analysis of morphological and functional MRI and their applications in radiation target definition and response assessment for adaptive RT
• MR Imaging quality improvement and quality assurance for MR-Sim and MR-Linac
• Clinical studies of implementing MR in RT as well as early evidence of efficacy
Keywords: MRI, MR-Linac, MR simulator, Functional Imaging, Radiotherapy
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