Explore · Movement & radiotherapy
A target that moves.
Move through an illustrated cycle, keep several positions in view, or let the symbol move. See why one snapshot cannot describe movement.
A movement concept, not a treatment or breathing exercise. The symbol, path and timing are invented. There is no anatomy, dose map, treatment margin or beam. Do not follow the animation as a breathing instruction.
One symbol / several moments
Not to scale
The circle stands in for a target whose position changes. The trace shows the vertical component of this invented path; it is not a breathing record. The grid has no units.
Reduced motion is on. Use the slider to explore individual positions.
A snapshot is one moment.
Some tumours move as the surrounding organs move with breathing. Imaging may be used to assess that movement. A 4D CT, where appropriate, can characterise different parts of a breathing cycle; it does not stop the target moving.
Keeping positions in view here illustrates why information from more than one moment can matter. The dashed circles are not a clinical motion envelope or a recommended margin.
A position needs to be reproducible.
For some treatments, a repeatable breath-hold helps manage movement or the relationship between the target and nearby tissues. The team checks whether the approach is comfortable and reliable.
This explorer does not train breath-holding or reproduce a patient’s breathing pattern. Follow the instructions given by your own radiotherapy team.
Timing is another way to manage movement.
With respiratory gating, treatment delivery is restricted to a selected part of the cycle and pauses outside that window. The technique depends on appropriate monitoring and checks.
No beam is shown here, and no gating window is calculated. Tracking and adaptation are other approaches in selected settings; this small illustration cannot choose between them.
The important difference
Measuring movement, making a position repeatable and timing delivery do different jobs. More elaborate technology is not automatically the right choice. The target, surrounding tissues, clinical plan, equipment and what a person can reliably manage all matter.
Real movement may be irregular, may differ between cycles and may include deformation. This smooth two-dimensional loop deliberately omits those complexities. It cannot assess the safety or accuracy of a real treatment.
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Sources & clinical review
Clinically reviewed and approved by Dr AlHilali on 4 November 2026.