Reply To: Vertical shifts from radiographic images

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Marko, thank you for your explanations. They were very helpful.

So my next question has to do with the final verification with the MV tangent portals. Should we shift based on these? My instinct says no, since these images do not provide orthogonal information. So what is the purpose for the MV portals? (1) To verify the relative position of the breast (target) relative to the MLC aperture and (2) to verify the position of the heart relative to the MLC aperture.  If these images indicate shifts >= 3 mm, what is your process? In your experience how often does this happen?