SGRT Forum

Do any centres using SGRT for SABR have SGRT tolerances of patient movement which would indicated if a mid CBCT would need to be taken? We're a little bit behind, and are just about to 'go live' with Lung SABR in free breathing (no compression or BH technique). We plan to take a post-CBCT intially to help develop our protocols as we build confidence, but is there anyone who has already undertaken this work who would be willing to share their findings? Our patients will be planned using 4DCT so we will have an idea of their 'normal' breathing, but how much movement is too much during treatment off zero before we should be looking at repositioning/rescanning?

Hello Suzanne,

we are treating Lung SBRT with SGRT, also in free breathing. Our current tolerances for this indication are 2 mm. Meaning in our SGRT System Align RT the patient can breathe 2 mm in any direction, before the system stops the treatment via the beamhold function. Most patients have a higher breathing amplitude/range during our 4D-CTs that are used for the treatment plans. Still our MPEs would like to not reach the max. registered breathing range during the treatment itself. So we instruct the patients beforehand accordingly to take calm and slow breaths. If a beamhold is triggered we usually first wait a little while, to see if the patient regains that breathing rythm and position. Sometimes we also advise the patient to take more shallow or deeper breaths. If that does not work, we re-do the CBCT. But that is only very rarely to the case.

During the starting phase, we also initially took a second CBCT post-treatment to verify our methods.
We also verify, that our PTV is large enough to accomondate every breathing phase, with a Motion View imaging before the first treatment.

best regards

We've been doing this for several years now and published our methods last year. Here is the link to our paper. https://pubmed.ncbi.nlm.nih.gov/32196944/

We also explored the dosimetric consequences of the intra-fraction motion if it had not been corrected and found that it doesn't matter a whole lot to the ITV dose. We presented this at ASTRO in 2019. https://www.redjournal.org/article/S036 ... 2/fulltext

Suzanne,
Our center has been using SGRT for several years now. Our PTV expansion from ITV for lung SBRT is typically 5 mm for free breathe patients, so we are comfortable using 3 mm as the tolerance in our SGRT system. We do not provide any coaching to these patients other than to "breathe normally." We do not use the auto beam-hold in this scenario because a patient's regular breathing will often take the "vert" out of the 3 mm tolerance. Our general workflow is to:

1) Set the patient up using the SGRT system.
2) Acquire a reference capture before the CBCT. After the reference capture we make note of the patient's "normal" breathing per the SGRT system. For example, the patient's "vert" may oscillate between -2 mm and + 2 mm.
3) Acquire the CBCT, perform the 6dof match and shift the patient.
4) Immediately acquire a new reference capture after the shift.
5) Initiate treatment. If the SGRT system indicates the patient has moved outside our 3 mm tolerance, we manually stop the treatment and re-CBCT.

In our experience, the SGRT system has worked great to monitor SBRT patients during treatment and provided us with additional confidence that we are delivering the proper treatment. Rarely do we need to re-CBCT (I don't have any hard data, but I would say less than 10%). When we do need to re-CBCT, the shift is usually on par with what the SGRT system is telling us.

Regards,
Brandon

Hi J.Zu,
What do you do to keep patients within the 2mm tolerance? We use chest compression, and even with a 3mm tolerance, I would say most patients exceed this tolerance at some point with their breathing pattern. Once in a while we will have a patient with excursions upward of 5mm in their natural breathing pattern (even with compression). As long as their breathing is such that they are within that 3mm threshold for a large enough fraction of their breath cycle, we will continue the treatment. If they do not return to 0 or inside that 3mm threshold at some point in their breath cycle, we will re-CBCT. We use 4DCTsimulation, and use a 5mm expansion on the ITV for our PTV.
Regards,
Juan