Avoidance sectors through a serial OAR
Stop the beam entering through the organ you are trying to spare, rather than asking the optimizer to undo it afterwards.
Built in·updated 2026-08-03
If a serial structure — cord, cauda equina, oesophagus — sits between the entry surface and the target over part of the arc, block that part of the arc, or drop the static fields aimed through it.
Why it works. Every objective is a compromise the optimizer negotiates. Geometry is not: a beam that never enters through the structure deposits no entrance dose in it, and the optimizer gets to spend its effort somewhere useful.
What it costs. Removing entry directions removes degrees of freedom, so coverage and conformity get harder. On a case where the OAR limit is the binding constraint that trade is usually worth it; on one where it is not, you have made the plan worse for nothing.
How to tell which you are in. Look at the beam entry geometry against the anatomy before the first optimization. If the OAR is not in anyone's entrance path, this is not your problem.
How to do it in
- Blocked structure
- the serial OAR
- Block type
- Directional / partial full blocking costs coverage
On a helical delivery this is directional blocking: mark the VOI as blocked or partially blocked and the planning system removes the beamlets that enter through it.
Full blocking is expensive — it removes every entry direction through that structure and coverage pays for it. Partial blocking, where a percentage of the entrance fluence is allowed, is usually the better trade on a case where the OAR limit is tight but not the only thing that matters.
On a fixed-cone or Iris delivery the equivalent is restricting the node set, which is a bigger decision and worth making before the first optimization rather than after.