Your independent check of an AP cervical spine plan finds the superior jaw 2 cm above the approved portal border, directly over the thyroid, while the inferior border sits exactly on the approved target margin and the monitor units match the plan. Department policy requires the delivered field to agree with the approved portal image before a fraction is treated. Which correction brings the field into agreement with the plan while keeping the prescribed target dose?

The jaws define the field edges, and the light field is used to indicate those edges when the system is properly calibrated. Moving the superior jaw to the approved portal border restores the planned field size; keeping the planned monitor units then delivers the planned treatment, since those units were calculated for the approved field. This is why the correction belongs in the collimation rather than in the output. Lowering monitor units to offset the oversized field does not correct the field outline and changes the dose. Shifting the whole field inferiorly moves both jaws, so the superior edge reaches the approved line only by moving the inferior edge 2 cm past the margin that already sat exactly on the target, creating a new mismatch. Raising monitor units after closing the jaw would increase the dose above the plan, because the planned units already correspond to the approved field size. Field size can affect output and scatter, but here the required correction is conformity to the approved outline. Compare the field with the approved portal image and resolve any mismatch before treating.