Adapting in Real Time: An Introduction to Adaptive Radiation Therapy

Adaptive radiation therapy (ART) is widely discussed in radiation oncology, yet it often remains somewhat abstract for many clinicians, as many training programs lack structured education in ART.¹ ART refers to the practice of adjusting a radiation treatment plan during the course of therapy to account for changes in a patient’s anatomy or tumor biology.² A helpful way to think about ART is to imagine it as a navigator guiding a long road trip. Even if you start with the best possible route, real conditions change along the way. Roads close, traffic builds, weather shifts, and without updated directions, you risk drifting off course. ART works in a similar fashion. Tumors shrink, patients lose weight, postoperative regions change, and organs shift and fill differently from day to day. These changes can meaningfully alter how a carefully designed plan performs, sometimes leading to underdosing of the target or unintended exposure to nearby organs at risk.¹ ART emerged in the late 20th century as an extension of image-guided radiation therapy, expanding the idea that imaging can do more than verify setup. It can also guide how the plan evolves throughout treatment.¹

There are 3 forms of ART: offline, online, and real-time adaptation.¹ Offline ART relies on images collected during treatment to identify consistent anatomical changes, with plan adjustments made before upcoming fractions. It is useful for addressing gradual or predictable shifts but does not respond to same-day variations.³ Online ART takes a more immediate approach by generating a revised plan right before treatment, incorporating that day’s anatomy directly into dose delivery. Real-time ART, made possible largely by MR-guided linear accelerators, goes even further by providing continuous imaging and allowing plan adjustments immediately before or even during beam delivery.³ This level of responsiveness is powerful, although it requires more time, expertise, and coordination.

Early clinical experience suggests that ART can improve target coverage, reduce dose to organs at risk, and enable safe dose escalation in selected scenarios.⁴ These benefits are especially relevant in disease sites with substantial day-to-day anatomical variability, including head and neck, prostate, pancreas, and thoracic malignancies.⁵ Still, widespread adoption has been slowed by the additional time, staffing, and coordination required for online and real-time adaptation.⁵ Although dosimetric improvements are well documented, high-level evidence demonstrating consistent long-term clinical outcome benefits is still emerging.

Overall, ART represents a shift away from static, pretreatment planning toward a more dynamic and individualized model of radiation delivery.² As imaging, automation, and adaptive planning tools continue to advance, future studies will be essential to refine patient selection, confirm clinical benefits beyond dosimetry, and establish consistent frameworks for routine implementation. ART is likely to play an increasingly central role in making radiation therapy more responsive and personalized over time.