Treatment Planning Considerations with MRI
Dose calculation and electron density
MRI does not provide electron density; CT remains the standard for heterogeneity correction. MR-only workflows use synthetic CT (from MR, e.g., Dixon or atlas-based methods) with validated HU/density assignment. Understand that dose calculation accuracy depends on correct density mapping.
Contouring with MRI
MRI improves GTV/CTV and OAR delineation. Know sequence choice: T1 post-contrast for enhancing tumors, T2/FLAIR for edema, DWI for cellularity, and dynamic contrast for perfusion. Be aware of geometric distortion affecting contour accuracy, especially near air/bone interfaces and at high field.
Plan evaluation
Understand how MRI-derived targets influence margins, and the importance of consistent registration. Consider that MRI signal does not equal dose; dose is computed on density maps.
Traps
Questions may imply MRI can replace CT for dose calculation without synthetic CT - it cannot directly provide electron density. Do not assume the brightest MRI signal is the target; contrast enhancement and edema differ. Distortion near sinuses/skull base can shift contours.
Rule of thumb
Use MRI for delineation and CT (or validated synthetic CT) for density; never assign dose based on MR intensity alone.
Worked example
In an MR-only prostate workflow, a Dixon-based synthetic CT is generated, validated against a CT for density accuracy, and then used for dose calculation while T2 MRI defines the prostate and OARs.