Advances in medical devices have allowed the use of CT, MRI, and PET-CT for the diagnosis of tumors and the detailed evaluation of the extent of lesions. For several decades, CT has been established as the gold standard modality for the treatment planning of radiotherapy, while MRI has emerged as a tool to evaluate the functional characteristics of tumors without radiation exposure. To further optimize precision radiation therapy, we should consider how functional images can be used in the workflow for radiation therapy. In this regard, MRI, as a modality without the need for a contrast agent, may allow more frequent scans and more detailed dose painting, such as increasing the dose to viable lesion parts while reducing the dose to less aggressive parts. Thus, a more personalized treatment based on precision radiation medicine might be realized. In recent years, MR-Linac systems (MRI integrated linear accelerator radiation therapy systems) have been applied in clinical settings by fusing MRI with Linac planning, and further development of radiation therapy utilizing MRI-derived functional images is expected. The use of MR-Linac techniques allows the characteristics of the tumor to be evaluated in more detail before treatment, and the treatment planning can be modified according to the position and characteristics of the tumor (which may change daily during irradiation) to avoid harming normal tissue. Compared with conventional cone beam CT, MR-Linac can offer MR images with much better contrast of soft tissue for image-guided radiation therapy, even when acquired at 0.35 T. A multicenter study of liver tumors using MR-Linac was recently reported. In current tumor imaging, various MRI sequences can be used to evaluate tumor functional information such as tumor heterogeneity, cell density, microenvironment, angiogenesis, necrosis, hypoxic status, and microstructure. In this article, we introduce state-of-the-art acquisition methods for MRI imaging, and discuss how the functional information obtained from these imaging methods can be useful for radiation therapy.
Keywords: MR-Linac; MRI; functional images; radiotherapy; tumor.