A patient is positioned for a lateral head-and-neck field, and the tabletop edge would clip the beam at the planned gantry angle. The therapist plans to adjust the couch before the first fraction without disturbing the tattooed setup. Which couch adjustment keeps the beam clear while the patient stays at isocenter?

Rotating the couch about its vertical axis while keeping the patient centered changes the tabletop’s orientation relative to the planned lateral beam, which can remove the edge from the beam path without intentionally shifting the patient away from isocenter. Rechecking the marks at isocenter confirms that the tattooed setup remains consistent after the adjustment. Altering the gantry angle can be a valid way to avoid a collision or obstruction, but it changes the planned beam direction; the stated problem calls for clearing the beam by adjusting the couch, not changing the planned gantry angle. Replacing the tabletop with a narrower indexed insert may provide clearance while preserving patient position, but it requires changing the treatment support setup rather than making the planned couch adjustment without disturbing the tattooed setup. Rotating the couch and relying on image guidance instead of the tattoos may produce an accurate image-based alignment, but it does not satisfy the stated requirement to preserve and verify the tattooed setup. Exam caveat: couch rotations can move anatomy away from isocenter if the patient is not centered on the rotation axis, so the pivot and rotation direction matter. Practical check: after adjustment, verify the tattoos against the room lasers at isocenter and confirm the planned beam clears the tabletop before treatment.