Treatment Delivery and On-Board MRI Guidance
MR-guided RT systems
Understand integrated MR-linac platforms (e.g., 1.5 T and 0.35 T systems) and the workflow: daily setup, on-board MRI, registration to reference, plan adaptation (online or offline), and delivery with real-time imaging. Know gating/tracking using cine MRI and how beam-on gating works.
Adaptation
Distinguish offline (library/plan-of-the-day) from online adaptive radiotherapy, where the plan is re-optimized on the day's anatomy. Understand contour propagation, re-optimization, and QA of the adapted plan. Time and resource demands are higher.
Safety and QA
MR-linac combines MR safety (Zone IV, implants) with RT safety (beam, MLC). Coordinate MR and radiation safety programs. Daily QA includes imaging geometry, output, and gating accuracy. Motion management: breath-hold, gating, and tracking.
Traps
Questions may confuse online adaptive with offline; or assume real-time MRI means no need for setup verification. Remember the magnetic field affects beam transport (electron return effect, dose at air-tissue interfaces) and that MR safety screening still applies.
Rule of thumb
On-board MRI defines the anatomy of the day; always verify registration and gating before beam-on, and re-optimize when anatomy deviates beyond tolerance.
Worked example
For a pancreatic MR-linac treatment, acquire daily MRI, propagate contours, re-optimize online when the target moves, and gate delivery to the tumor position using cine imaging.