Optimizing lymph node metastasis imaging in MR-guided radiotherapy for oligometastatic prostate cancer.

MR-guided radiotherapy (MRgRT) using MRI integrated linear accelerator (MR-LINAC) is a critical advancement in prostate cancer (PC) treatment. Leveraging the superior soft tissue contrast of MR imaging (MRI), along with online adaptation capabilities and real-time MRI for advanced motion management, MRgRT has demonstrated efficacy in localized PC irradiation. Recent studies have demonstrated that MRgRT can effectively treat individual lymph node metastasis (LNM), with preliminary results showing excellent patient tolerability and safety as a novel metastasis-directed radiotherapy for oligometastatic PC (omPC). However, current LNM imaging on an MR-LINAC is limited and suboptimal, creating reliance on diagnostic MRI and positron emission tomography (PET) for differentiation, registration, contouring, and adaptation. The inherent uncertainty and errors within these processes impair the accuracy and precision of LNM irradiation in MRgRT. This review aims to provide an overview of the current imaging techniques employed in clinical MRgRT practice for omPC, highlighting the unmet clinical needs due to the limitations of LNM imaging using an MR-LINAC. Furthermore, it reviews promising advanced LNM MRI techniques potentially transferable to MR-LINAC. Lastly, it discusses future developments in LNM imaging on an MR-LINAC and its potential as a comprehensive one-stop imaging solution for future omPC MRgRT.