Adapt-to-position or adapt-to-shape
One reuses the plan's shapes against today's position; the other re-contours and re-optimizes. The difference is roughly half an hour.
Built in·updated 2026-08-03
On an MR-linac the daily decision is which adaptation to run.
Adapt-to-position optimizes the reference plan's beam weights or shapes against a rigid registration of the planning CT to today's MR. Contours cannot be edited. Reported overall session times are around 46 minutes for abdominal SBRT.
Adapt-to-shape deformably propagates the contours, lets you edit them, and re-optimizes from scratch. It handles genuine anatomical change rather than displacement, and sessions run 60–90 minutes.
Full online re-planning is the most robust of the options, and the choice is a trade between the time spent adapting and the dosimetric gain — a trade that is different for a bladder that filled than for a patient who is simply lying 8 mm to the left.
Why it belongs in a write-up. A challenge case is planned once, offline, so this never appears in the score. It appears the moment someone tries to deliver the plan you wrote, and a plan that only works at the planning anatomy is a plan that will be adapted away on day one.
Where the numbers come from
- Adaptive radiotherapy: the Elekta Unity MR-linac concept
Clin Transl Radiat Oncol · 2019
ATP vs ATS workflows and their trade-off between time and dosimetric gain
- A new workflow of the on-line 1.5-T MR-guided adaptive radiation therapy
Jpn J Radiol · 2023
ATP ~46 min for abdominal SBRT; ATS sessions 60–90 min