Central nervous system tumors

MRI is the standard imaging modality used to assess brain and spinal tumors given its excellent anatomic soft tissue visualization and numerous additional methods that can be used to investigate physiology (e.g., diffusion, perfusion, spectroscopy). Typically, brain tumors are assessed with MRI only at certain time points, such as before the start of radiation therapy (RT) to assist with treatment planning, and again approximately 1 month after the completion of RT to assess the tumor’s response to therapy. MR linac devices have unique benefits for central nervous system (CNS) tumors, including allowing for more frequent imaging of tumors, such as during treatment. In the brain, MRI-guided radiation therapy (MRIgRT) provides daily imaging of evolving tumors, which cannot be visualized using cone-beam CT guidance due to poor soft tissue contrast and greater temporal information of the tumors than discrete stand-alone MRIs. For the spine, MRIgRT acquires clear isotropic images of tumors in relation to the spinal cord, cerebral spinal fluid, and nearby moving organs at risk. This visualization allows for treatment setup reassurance and the possibility of adaptive radiotherapy based on moving anatomy in difficult cases. Another major benefit of MRI is its ability to capture additional information beyond anatomical changes depending on the MRI contrast mechanisms being assessed. It is possible to obtain MR relaxometry, diffusion, perfusion, and spectroscopy (multiparametric MRI) during RT using MR linac devices. These techniques are known to correlate with physiologic changes in the tumor such as cellularity, necrosis, and metabolism, and serve as early biomarkers of chemotherapy and RT response correlating with patient survival. This chapter discusses the potential use of MR linac devices for anatomic and physiologic adaptive radiotherapy for treating brain and spine tumors.