Transforming radiotherapy: the benefits of Magnetic Resonance Imaging (MRI) in precision oncology

Both MRI and CT have earned their place as essential imaging modalities for radiation oncologists, each offering unique strengths that complement the other. MRI offers distinct advantages for adaptive radiation therapy planning, particularly for tumors.¹ Its superior soft-tissue contrast and detailed anatomical visualization can enhance treatment precision, making it a compelling option for tailoring radiation treatments to individual patient needs. As the field evolves, incorporating MRI insights into adaptive planning may improve outcomes and refine treatment strategies.

Complimentary to CT, MRI’s superior soft-tissue contrast and target delineation may improve patient outcomes in a variety of cancer types.^1,2

Studies also indicate that MRI often results in smaller, more precise anatomical structure volumes (pelvic, prostate, rectum, cervix, and uterine body), reflecting the superior soft-tissue contrast when compared to CT imaging.³ This is especially the case in regions like the parotid gland and cerebellum, where edge detection is crucial.³

Another advantage is that MRI uses non-ionizing radio waves, eliminating the risk of additional radiation exposure associated with CT.¹ This aspect is particularly beneficial when working with pediatric patients and in cases in which repeat scans are needed during treatment. Additionally, MRI allows for flexible slice orientation, which can be aligned more naturally with patient anatomy.¹

Current CT guidance in radiotherapy has limitations, particularly in the accuracy of gross tumor volume (GTV) delineation for some disease sites. Studies have shown that variability in GTV can introduce errors comparable to those caused by daily setup uncertainties. This inconsistency has been described as a major weakness in radiotherapy precision, affecting the overall effectiveness of treatment. Improving GTV accuracy is essential for enhancing radiotherapy outcomes.^4-6

Many studies have shown that anatomical structures such as the pelvis, prostate, rectum, cervix, and uterus often appear smaller on MRI compared to CT.³ This difference reflects MRI’s superior ability to capture soft-tissue contrast. In head and neck imaging, the size of certain structures varies between the two modalities—some are larger on CT, while others, like the parotid gland and cerebellum, are notably smaller on CT. This highlights MRI’s enhanced capacity for detecting tissue boundaries, offering clearer, more precise soft-tissue delineation compared to CT.³ 

While steep dose gradients protect nearby organs, inconsistent delineation on CT can result in geometric misses and, ultimately, recurrence of disease. Even with on-board CT image guidance, systematic delineation errors can persist. ⁶ Given the limitations of CT, particularly the variability in GTV delineation and the potential for geometric misses, MRI offers a valuable alternative by providing more consistent and accurate soft-tissue contrast. This precision helps minimize the risk of systematic errors and improves the accuracy of radiotherapy planning, reducing the likelihood of recurrence and protecting surrounding organs at risk.³

In prostate cancer, for example, research shows that semi-weekly stereotactic body radiotherapy with iso-toxic focal boosting results in minimal acute genitourinary and gastrointestinal side effects.⁷ Studies demonstrate a clear dose-response relationship, indicating that targeted dose escalation at the tumor site reduces local failure and lowers rates of regional and distant metastases.⁸

Furthermore, incorporating MR-guided focal boosts into a radiotherapy regimen has proven effective without significantly increasing toxicity, ensuring better tumor control while protecting healthy tissues.⁹ The Focal Lesion Ablative Microboost (FLAME) study highlights that adding a focal boost significantly improves biochemical disease-free survival for patients with localized intermediate- and high-risk cancer, all while maintaining quality of life.^10-11 This innovative approach represents a promising advancement in radiotherapy, paving the way for improved patient outcomes in cancer care.