5.6 - Action Thresholds

What is meant by an ‘action threshold’ for position error?
How is this threshold determined?
How is this related to PTV margins used in RT planning?

Action Thresholds

An action threshold is the amount of discrepancy between the planned treatment and actual position on treatment verification imaging that is permitted before a shift is applied. It may vary depending on the treatment site, treatment intent and on the treatment verification method used.
If consistent shifts are applied, then the treatment plan is altered slightly to account for the changed position. Major differences in positioning on treatment verification may be due to altered patient anatomy between planning and treatment, often requiring resimulation.

Determination of Action Thresholds

Action thresholds are set by individual departments, and are dependent on the treatment sites and treatment intents. Individual machines may have different thresholds depending on their treatment verification techniques.

  • For example, a linac with an OBI device may have a smaller action tolerance than an older linac with portal films only

Relation of Action Threshold to PTV margin

The PTV expansion from the ITV is the external margin, which compensates for uncertainty due to patient setup. If a treatment can be made more accurate by using treatment verification techniques - particularly advanced measures such as OBI or cone-beam CT - then the expansion can be reduced accordingly. This allows increased sparing of normal tissues.