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.