During weekly QA on the treatment linac, film shows that the radiation field and the light field are the same size and shape, but the light field sits a few millimetres to one side of the radiation edge every time. The offset appears at every couch height and measurement distance, and the jaw settings match yesterday's record. Which component should be adjusted first to restore light and radiation field coincidence?
Select an answer to reveal the explanation.
Short Explanation
Because the fields stay the same size while the light sits consistently to one side, your jaws are likely fine, so adjust the light-field mirror first. Like a misaligned projector, the mirror can shift the light image while the radiation field stays put.
Full Explanation
The light-field mirror in the treatment head directs the field light, so if it tilts or slips, the light image can move sideways while the jaw-defined radiation field stays in place. The unchanged field size, consistent one-sided offset, and persistence at different couch heights and measurement distances point to a misalignment of the light-field projector. Recalibrating the jaw position encoders would change how the jaw opening is displayed, not correct a relative displacement between the light and radiation fields. Releveling the treatment couch would not fix the projector’s alignment, and the offset at every couch height argues against couch sag shifting the film. Adjusting the room alignment lasers would change the setup reference, not the light field projected from the treatment head. A displaced film cassette can mimic this symptom, so repeat the measurement before making an adjustment. As a practical check, rotate the collimator 90 degrees and repeat the measurement: a head-mounted misalignment will rotate with the collimator.