Role of MR-guided Radiotherapy (MRgRT) in Colorectal Cancer

Purpose of Review: The current standard treatment for locally advanced or metastatic colorectal cancer often includes radiotherapy delivered under computed tomography (CT)–based image guidance. Magnetic resonance–guided radiotherapy (MRgRT) offers a substantial improvement in target and adjacent organ visualization and allows for real-time imaging throughout treatment to ensure target accuracy with each fraction. However, this technology is in its relative infancy, with many open questions regarding how to maximize the potential of this new treatment modality. We review the literature and share our institutional experience to highlight the strengths and limitations of MRgRT for treatment of colorectal cancer. Recent Findings: MRgRT is safe and effective in both the locally advanced and metastatic settings. Dose can be safely escalated in locally advanced cases, potentially improving rates of pathologic complete response. Functional imaging offers insight into treatment response dynamics, opening the possibility for nonoperative management of select cases. Liver metastases can be treated to ablative doses with high rates of local control. Summary: MRgRT has the potential to shift the paradigm of treatment for locally advanced and metastatic colorectal cancer. Improved target accuracy and real-time gating allows for dose prescriptions beyond what has been achievable with CT-based imaging, allowing for higher rates of tumor control with no significant increase in toxicity. Future work will focus on optimal dose fractionation schemes and functional radiographic assessment of tumor response.