SGRT Forum

I have been using AlignRT with VARIAN linacs for over five years. This is my first experience using an Elekta linac (VersaHD) with AlignRT. I am running into issues that are not an issue with VARIAN linacs, such as being able to monitor the patient during a CBCT and having great difficulty monitoring the patients during treatment.

In VARIAN linacs, I am able to monitor the patient even though the kV and source are extended and blocking the AlignRT cameras. However, with the Elekta linac the blockage of the cameras by the kV source and panel (and the gantry) make it impossible to monitor the patient for motion during CBCT acquisition. The cameras are not able to see sufficient surface, so all the RTDs are frozen.

In VARIAN linacs, even if the kV panel and source are extended, it is possible to monitor the patient during treatment delivery for most sites. There will be breaks at certain gantry positions, but they are only temporary. With the Elekta linac, there are too many gantry positions for which the panel and source prevent the system from seeing sufficient surface, so we must push both in prior to treatment. The drawback of the Elekta linac versus VARIAN linacs is that with the Elekta linac the therapists must go into the room and manually retract the source and panel (this can be done from outside the room in VARIAN linacs). I am also getting the feeling that the field of view of the AlignRT cameras is reduced in an Elekta linac compared to a VARIAN linac, particularly if the hexapod is used, as it lifts the patient vertically by about 5 cm.

Patient monitoring seems to be greatly affected by the height of the patients, particularly when treating the pelvis. I assume the issue has to do with the fact that taller patients are pushed farther in toward the gantry which exacerbates the blocking of the patient by the gantry.

If there is anyone is the SGRT community that has been using AlignRT with Elekta linacs, will you please share with me how you have overcome the issues I mentioned above? I am concerned that we will not be able to use AlignRT for SRS/SBRT cases.

Thank you,

Hi Jose,

We use AlignRT with Elekta in Birmingham, England. You are correct in as much as you certainly need to put the XVI tube and panel away. If you don't they will block the camera view. This is true for CBCT acquisition and treatment delivery. And you can see it happen by the light projection being on the back of the panel/tube rather than the patient, as well as the deltas changing erratically. This is all we've known as we have only ever used AlignRT with Elekta and it would be nice to see the next generation Elekta Linacs have a remote camera and panel retraction option.

So the only thing it really creates for us is a blip in monitoring the patient when acquiring a CBCT. But you can see when the cameras are able to see the patient based on the projection so when we have known camera block during acquisition, we watch the deltas and if they jump out of tolerance we don't panic. As soon as the known camera block is over (maybe 10 seconds later), if the deltas come back in to tolerance, then the patient hasn't moved. If they stay out of tolerance and you are sure the camera block is over, then we interrupt as it is likely that the patient has moved.

We also optimise our ROI for VMAT deliveries to let AlignRT see more of the patient. This avoids blips at certain gantry angles.

I can see how the height of the patient could reduce the camera view as it can create a shallower angle (depending on patient anatomy) but whether the patient is further in or out of the gantry shouldn't make a difference as the area you are treating will still be at the isocentre.

In summary:

Don't worry if there's camera block when acquiring a CBCT - just ensure the deltas move back when the camera block is over.
Put the tube and panel away when treating (or you'll have to deal with camera block issues during treatment which we don't feel is the best way to use the system).
Fingers crossed for more automation of panel and tube on Elektas part in the future.

I hope this help.

Ben Allen

Hi Jose,

I came from a Varian show site and now work in a full Elekta environment at UC Davis. Things operate slightly different with the equipment however I have not experienced major differences in the utilization of AlignRT with Elekta vs. Varian. We treat SBRT patient routinely in our clinic and the therapist have a great workflow for utilizing Hexapod, AlignRT and CBCT together. Here are the steps for a typical SBRT patient:
1) Align patient in room using AlignRT. The therapist really like to utilize postural alignment for this as it helps with determining whether the patient is in the proper position within the vaclok.
2) Perform Motion view if requested by the physician.
3) Perform CBCT and make adjustments. We have created a policy within our department to identify how much rotational shift we would actual like to allow with Hexapod. If over that value the therapist go back into the room and reset patient (this is a very rare occurrence). The reason for setting this policy in place because as you mentioned the amount of translation shift that the precise couch top makes in relation to the hexapod can be pretty significant and concerning.
4) while waiting for the physician to approve shifts and images therapist #2 will go into the room and retract the imaging arms and get the gantry into position for treatment. Most of the time the driving therapist will have sent shifts by this time and it gives the therapist a great opportunity to apply shifts from in the room and verify clearance.
5) Once shifts are made, a new SGRT capture is taken. Beam hold can be turned on at this point and patient gets treated.

During CBCT we do see occlusion somewhere starting around 15 deg- 25deg and 340ish on the opposite side. Once we start treatment we do not seem to have issue with gantry occlusion. This is very similar to what I would see occur when using Varian equipment as well. I have found it helpful to create an upside T, making sure to include enough laterally as well.

Overall we have not found that retracting imaging arms adds a significant amount of time to our treatments. Our SBRTs are typically scheduled in a 30 min time slot.

I agree with Ben's summary as well.

Kind Regards,

Jotsna Singh

Thank you Ben and Jotsna for your responses.

It seems that we have an issue that neither one of you is experiencing. When we take a CBCT, the issue is not that we occassionally get blips in the RTDs: the cameras are unable to see the patient at all during the entire acquisition (not sufficient surface detected) so there are no RTDs to monitor. Between the gantry, the XVI panel and source, and the hexapod frame, the entire FOV is blocked. Do you have recommendations for where to place the hexapod frame or do you remove the frame during the acquisition?

Thank you,

Hi Jose, Blip of RTD's at particular gantry angles can be fixed (even with hexapod frame) by increasing the ROI(Little away from Treatment iso) where Pod 3's projections are not block on patient skin. Thank you Ben Allen for your nice suggestion to increase the ROI. As i said in my previous comment appropriate ROI will fix this blip of RTD's. I think we need to accept the benefit of kV detector at 60cm from iso, through which we can acquire CBCT even at couch lateral 10cm. Once again Thank you all.

Check if reply is posted

Hello Jose,

we have been using the Align RT System with our Elekta Linacs (Versa/Synergy Models) since January 2020 and went markerless on all sites a couple of weeks ago. It is indeed difficult, if not impossible, to monitor the patient during the CBCT/XVI imaging with all panels/source extended. Due to that the Panel/Source have to be manually retract by the therapist. There seems to be no way around this, if Elekta does not decide to increase usability by retracting the panels/source remotely in their next linac installments.

Regarding the height - we are using The ORFIT Industries sagitillt board for prone breast treatments and that has been the only case where the Align RT FOV reached its limits, as the patients are liftet at least an additional 15-20 cm above the regular table. And only then the FOV showed not detected areas/clipping on the back of the larger patients. Every other indication or positioning device has not come close to the FOV edge. One other lightly similar case was isocentric table rotation of 90/270° and even there we had only minimal clipping. As Anandakri mentioned it is dependent on the height of the installed Align RT Pods.

If you make a Reference Capture for a Pelvis Patient - how far does the Capture reach into the Gantry/how much of the patient is shown? At our sites it is, even with maximum cranial positioning due to patient length, more than enough to use sucessfully monitor every patient. If you have enough of the patients body on the Reference Capture, maybe try to enlarge the Pelvis ROI to the cranial body parts above the Pelvis. This might seem controintuitive but has helped us a lot with almost completely preventing beam holds due to the ROI being blocked by parts of the Gantry. If needed I could forward you an example of our ROIs look like. Just let me know.

best regards,

J. Muenster