Contouring the beam's way in
Drawing a structure along the entry path and constraining it hard is not a style — it is what you do when your machine cannot remove the direction geometrically.
Built in·updated 2026-08-20
Ask three planners how they stop dose entering through an organ and you get three answers. That is not sloppiness. They are describing what their machine lets them do.
The optimizer only controls what has a contour. If you want less dose along the path a beam takes in, and there is no structure there, that region does not exist to the optimizer. It is not being permissive — it was never asked.
Three tools for one job
| The machine has | What you do |
|---|---|
| Helical tomotherapy — directional block | Forbid entry through a named region. Geometric; no structure constraint involved |
| VMAT with jaws — partial arcs | Remove the gantry sector. Also geometric |
| Halcyon — no jaws, dual-layer MLC only | Draw the structure and make the optimizer close the leaves |
The Halcyon case is the clean example because the gap is documented. An oblique anterior field is useful for a breast SIB and the machine cannot collimate one, so an avoidance structure was scripted through the Eclipse API for the specific purpose of forcing the leaves shut when facing the organs at risk.
Read that sentence twice. The structure is standing in for hardware. It is not a better idea than a block — it is what is left when a block is unavailable.
Why nobody publishes the numbers
Look for a standard and you will not find one: how far up the path to draw, how thick, what dose limit, what priority. There is no consensus because there is no single technique here. Each version is shaped around a particular machine's particular gap, and the gaps differ.
So when a colleague's values look nothing like yours, the useful question is not who is right. It is what their machine could not do.
What it costs
A path structure pushed hard does not delete dose, it moves it. The optimizer reroutes through whatever directions are left, and the bill arrives as monitor units, as treatment time, and — if you keep tightening — as target coverage.
Two things worth checking before you accept the plan: whether MU rose out of proportion to what you gained, and whether the coverage you are quoting still holds at the edge nearest the structure. A path constraint that saves an organ and quietly costs D95 has not made a better plan.
On a scored case
Write down which tool you used and why, because the score cannot see it. Two plans with the same DVH — one using a directional block, one using a contoured path on a machine without one — are the same result reached from different starting hands, and the per-vendor recipes are where that difference belongs.
Where the numbers come from
- Scripting a planning avoidance structure to perform simultaneous integrated boost breast on Halcyon linear accelerators
Physica Medica · 2024
Halcyon has no jaw collimator, so an oblique anterior field cannot be formed. An avoidance structure was built through the Eclipse Scripting API to force the optimizer to close the leaves when facing organs at risk. Abstract only — the paper is paywalled, and no constraint values are quoted here because none were read.