SGRT Forum

Hi.  We use AlignRT for our DIBH patients and when we have a monoisocentric SCF field, we apply a 90 degree floor twist to the SCF field.  I have noticed that when AlignRT is monitoring in the SCF position, it often wants to apply a 3 or 4mm lateral shift.  At first I thought it was because the patient must have moved but we see it so often that I immediately started to take the floor back to 0 to see what happened.  The shift requirement disappeared.  I am not overly concerned as it's a minimal shift but I don't understand why this happens.  Could it be the fact you only have good coverage from 1 pod rather than all 3 so there's a slight discrepancy?  Has anybody else experienced similar?



Thanks,



Thanks,

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

Best Regards

Mike

Thanks for your reply.  I am inclined to agree that it's an ROI issue but as they are all left sided, the ROI is still visible from the camera at the foot end of the bed.  I might have a play on the next one with some treatment captures and see what the cameras can actually see.

The reason we have a floor twist for the SCF is because of the collimator angle on the tangential.  We put a 90 degree kick on the floor and angle the gantry to match the previous collimator angle (both the tangs and the SCF are also half beam blocked).

Cheers,

Ben