It can take a bit of time to get used to using OSMS, but at our center we find it very efficient, accurate and helpful to setup and monitor/gate our DIBH patients. We set our DIBH tolerances to 3mm for the translations (lat, Long, vrt), and 3° for the rotations (rot, roll, pitch). We first set up our patients using the FB (free breathing) DICOM from the planning CT, and correct for the translations and rotations as close to 0 as possible. Then we switch to the DIBH DICOM from planning CT, and ask our patients to breathe in and hold their breath so that the vertical is within the 3mm tolerance. It is important that they can hold their breath within this range, as this has been calculated in planning. If patients are unable to consistently hold their breath within this range, they may not be suitable for DIBH. We practice this during the planning CT to ensure it is reproducable during treatment. The amount they breathe in is calculated in planning, so when we set up the patients in FB first, they should be able to breathe in and hold their breath within the set vertical DIBH DICOM. Once they breathe in correctly, we may make minor adjustments to the table for the long and lat if necessary, but certainly not the vertical. We leave the room once we are pleased with the setup, do daily portal MV imaging on DIBH, and confirm the chestwall/bony anatomy and breast contour. Any small shifts after matching get made while having the patient breathe in, and we take a new reference at that moment which we then use to monitor and gate our patients for their treatment that day. We rarely have any issues this way during the DIBH
treatment. We find it is important to properly coach the patients during the planning CT on how to consistently breathe in and hold their breath. i.e. they should not breathe in lifting their entire chest or body, because then they may actually move themselves out of tolerance.
Another reason why the mag could go out of tolerance is sometimes with IMRT DIBH breast plans, it is possible that the gantry can block some of the OSMS pods/cameras. It helps to have an ROI that covers the treated breast, but also extends a bit beyond the midline, and wrapped underneath the non-treated breast. This way the pods have a slightly larger area to observe without going out of tolerance. If you are uncertain whether the patient actually moved after a few treated fields, or whether the mag is out of tolerance due to camera blocking, you could drive the gantry to 180° or 0°, have the patient breathe in, and check the OSMS values again. If it is within tolerance then, it is most likely due to camera blocking. Try adjusting your ROI. In some difficult cases, we may take a new image to confirm the anatomy and shift if necessary.
I hope this helps! Feel free to ask for anything else. Success! Greetings, Floortje