The adaptive problem
Bags, baseplates, headrests, couch overlays — and frames.
Online adaptive replans on the patient's anatomy alone, on the day's images.
Above 3%, hardware is supposed to be modeled. An adapted plan has no step for that.
This applies to every immobilization device, not only to frames. The question for an adaptive program is not whether the hardware attenuates, but whether it attenuates little enough to ignore.
The measurement
Eleven patients, three separately manufactured units. Single-beam attenuation through the frame was measured at 6, 10 and 15 MV, away from the side rails, and the dosimetric effect was evaluated on the patients' own plans.
- Under 2% single-beam attenuation at 6, 10 and 15 MV.
- 1.1% mean target D95 loss attributable to the device, with every patient inside 3%.
- −928 HU on the planning CT. Air is −1000.
One exception. Beams through a side rail exceed 3% at 6 and 10 MV. Avoid rail angles in IMRT planning where geometry allows; VMAT generally does not require this.
Data on file, Stabilix. Available on request.
What this means for your physics team
No device model in the treatment planning system, and no correction step in the adaptive workflow. The frame is in the planning CT like any other object in the room, rendering close to air, and the plan that is adapted on the day is delivered through the same frame it was planned in.
The attenuation data package has the method, the per-energy measurements and the per-patient D95 analysis, for your own review before any evaluation. Request it →
The rest of the frame
Low attenuation is the price of entry for adaptive, not the reason to use a frame. The reason is that the frame holds the patient: rigid restraint of the body and frame-mounted compression for the diaphragm, at 9.1 lb, on any couchtop. The system →
Questions
Why does attenuation matter more for adaptive radiotherapy?
A conventional plan can model a device in the beam path, or the planning system can account for it in the CT. An online adaptive plan is recalculated on the day's images and the patient's anatomy alone; there is no step in which hardware is modeled. The device therefore has to attenuate little enough to be ignored.
How was the attenuation measured?
Single-beam attenuation through the frame was measured at 6, 10 and 15 MV, away from the side rails, across three separately manufactured units, and the effect on target coverage was evaluated on eleven patients' own plans. The method and the data are in the attenuation data package, available on request.
What about beams through the side rails?
Beams through a side rail exceed 3% at 6 and 10 MV. Avoid rail angles in IMRT planning where geometry allows; VMAT generally does not require this.
Does the frame need to be in the planning CT?
It is in the planning CT like any other object on the couch, rendering at about −928 HU. No separate device model is needed.
Is the data peer reviewed?
The attenuation measurements are data on file at Stabilix and are available on request to healthcare professionals. Compression figures quoted on this site derive from the published literature on comparable rigid frame systems.