Precision is paramount in radiation therapy for brain tumors to ensure effective treatment while protecting healthy tissue. Magnetic resonance imaging (MRI) has become an essential tool in treatment planning, offering exceptional soft tissue contrast, superior anatomical detail, and functional imaging capabilities. When combined with other modalities such as CT and PET, MRI enhances treatment plan accuracy, enabling highly personalized approaches that align precisely with both tumor and normal tissue anatomy.
Importantly, relying on MRI as the central planning modality also reduces systemic registration errors that can occur when fusing CT and MRI datasets. Such errors – often measuring 2–5 mm – stem from differences in patient positioning and the intrinsic contrasts of each modality.1,2 By minimizing these inconsistencies, MRI-based workflows improve confidence in target delineation and help reduce the risk of geographic miss.1 In practice, this means greater consistency in anatomy mapping across repeat scans, as MRI-to-MRI registration shows less variability than CT-to-MRI.1 The result is a more reliable technical foundation for adaptive treatment strategies.
Unlocking MRI benefits for brain tumors
MRI offers precision in defining treatment targets and minimizing radiation exposure to healthy tissue.3 In comparative studies, MRI-guided planning produced significantly smaller clinical target volumes than CT-based planning (p = 0.02),4 underscoring its potential to spare healthy brain structures while maintaining full tumor coverage – an especially critical consideration in neuro-oncology. Clinical data in other tumor types reinforce this principle: MRI-based delineation has been shown to significantly reduce target volume compared to CT, with downstream benefits in reducing treatment-related toxicity.5 By providing superior boundary definition and reducing reliance on CT fusion, MRI not only refines treatment plans but also strengthens the overall workflow that underpins safe, accurate radiotherapy.
When compared to CT, studies indicate that MRI yielded incremental, therapeutically relevant information in 59% of patients.6
Field size, related to the irradiated area, is critical to reduce toxicities while treating small metastases. In six cases, MRI's superior border definition reduced field sizes by 15% to 66%, while in four cases, it revealed larger tumor extents, necessitating increased field sizes.6 Even when treatment plans remained unchanged, the accuracy of MRI gave clinicians greater confidence.6
With exceptional soft tissue contrast and advanced functional imaging – including diffusion-weighted imaging (DWI) and dynamic contrast enhancement – MRI excels in differentiating tumors from healthy tissue in areas where CT reaches its limits. In regions such as the posterior fossa and cervical spinal canal,6 MRI is indispensable for optimizing CNS tumor treatment through precise and efficient simulation and planning.
MRI’s role in monitoring and responding to change
MRI’s high-resolution imaging enables early detection of clinically significant anatomical changes during treatment – changes that may not be visible on other modalities. In patients with brain metastases, 41% required treatment modifications within 7 days of an MRI simulation, with this figure rising to 78% between 8 and 14 days7 – highlighting the importance of having dedicated MRI’s within the radiotherapy department. In pediatric craniopharyngioma, 35% of patients required intervention during treatment due to cyst changes, including both field enlargement and reduction.8 Repeat MR imaging can lead to vital clinical decision-making within the first 1–2 weeks.
These findings underscore MRI’s critical role in ensuring treatment accuracy throughout the course of therapy. Its superior soft tissue contrast and ability to delineate tumor boundaries help maintain precise targeting while protecting healthy brain structures, even in regions where CT images are degraded by bony artifacts.5 In some cases, this detailed visualization supports offline adaptive planning decisions, allowing clinicians to refine target volumes or adjust treatment fields as anatomy changes.7-9 Emerging MRI-based LINAC systems streamline offline adaptive planning by offering real-time imaging of edema, resection cavities, and tumor dynamics.⁴ These advances enable immediate plan adjustments, reducing reliance on standalone diagnostic MRIs and improving treatment outcomes.9
Consider MRI for effective simulation and planning in the brain
As radiation therapy evolves, MRI continues to set new benchmarks for brain tumor simulation and planning. By integrating structural and functional data, such as DWI and dynamic contrast enhancement, MRI provides critical physiologic insights alongside superior soft tissue contrast.10 These capabilities enhance target volume delineation, improve confidence in treatment decisions, and help ensure the highest level of accuracy in CNS tumor care.3,10 With its ability to capture both the anatomy and physiology of the tumor and surrounding tissue, MRI remains an invaluable tool in advancing personalized radiation therapy.6