Ring structure for dose fall-off
A shell a few millimetres outside the PTV, pushed hard, to force the gradient down instead of hoping for it.
Built in·updated 2026-08-03
A ring is a hollow shell built from the PTV: expand by 2 mm, expand again by 15–20 mm, subtract the inner from the outer, then subtract the PTV itself.
Give it a max-dose objective around 50–60% of prescription with a high weight. The optimizer cannot satisfy it and still spill dose, so the gradient outside the target steepens.
Why it works. Conformity and gradient indices are measured on isodose volumes, not on structures. Nothing in the objective list refers to the space just outside the PTV unless you put something there, and the optimizer will happily leave dose in a place nobody asked about.
Where it goes wrong. Too tight a ring fights coverage at the PTV edge and you lose more on D95 than you gain on GI. Start at 2 mm from the PTV and only move in if coverage is holding with margin.
How to do it in
- Shell / normal-tissue objective
- needs a Precision planner to fill in exact control names unconfirmed
- Contoured ring
- (PTV+2mm+18mm) − (PTV+2mm) for directional fall-off only
- Max Dose on the ring
- 50–60 % of Rx
This recipe needs a Precision planner. The first draft claimed Precision had neither automatic shells nor a normal-tissue objective, and it has both. Use Correct it above rather than trusting what follows.
Precision provides shell structures and a normal-tissue objective, so the usual route is to use those rather than to contour a ring by hand.
A contoured ring is still the answer when you want the fall-off in one direction and not another — an automatic shell is concentric, so it cannot spare posteriorly while allowing spill anteriorly.
Whichever you use, watch the target coverage objective on the same run: if D95 starts falling, the constraint is too close to the target, and moving it out is the fix rather than dropping its weight.