Michael Tallhamer, MSC, DABR
Do you see it regardless of the 90 degree rotation direction? I assume you are ensuring the rotation during planning is compliment to the side you are treating (270 couch for left breast or couch 90 for right breast in IEC 61217 ) to ensure the treated breast/CW is facing the foot of couch camera and not away from it.
Is the patient on an incline board or relatively flat (i.e. is there alot of self shadowing impacting how much ROI the camera sees in this orientation)?
My first kneejerk response would be that the 90 degree rotation is limiting the camera’s capability of seeing your ROI. That being said that would be fairly easy to test under the clinical orientation of a standard patient.
The next thing I would look at is the size of your ROI. I have seen folks use an ROI that covers both breasts and with an off axis isocenter on a breast patient that means with a 90 degree couch rotation the portion of the ROI that is far from the isocenter could be the majority of the visible ROI from the camera’s perspective. That means the ROI that is far away from the isocennter potentially has a less focused speckle pattern or is dominated by the contralteral breast which could be some of what causes the shift at this couch angle.
I would recommend not using the 90 degree couch kick as we don’t routinely do this for our single isocenter breasts and they seem to work well with the half beam geometry controlling the matchline. It isn’t readily apparent to me why the 90 degree rotation is needed but that may just be my lack of experience with this technique