A 6 MV plan delivers the prescription to a midline target at 10 cm depth with a 10 x 10 cm open field and 180 monitor units. The physician then widens the treatment field to 20 x 20 cm at the same energy and depth, and the record-and-verify entry still shows 180 monitor units. Which beam parameter must be recalculated before the plan is approved for treatment?

The field-size output factor must be recalculated because it accounts for the change in dose per monitor unit when the open field widens from 10 × 10 cm to 20 × 20 cm. At the same energy and depth, a wider field generally produces greater dose per MU through increased phantom scatter; leaving the entry at 180 MU without updating the field-size-dependent calculation can therefore alter the delivered dose. An inverse-square correction is relevant when source-to-target distance changes, but no distance change is stated, so it does not address the changed field. Recomputing MU from the prescription and depth-dose data while retaining the original 10 × 10 cm output factor is incomplete: it carries forward the factor for the old field and misses the changed field condition. The tissue-maximum ratio is also a standard dose-calculation quantity, but using a reference-field value at 10 cm does not account for the new field size; the target depth and energy are unchanged. Exam caveat: field-size effects may be represented through an output factor, scatter factor, or commissioned dose-calculation model, depending on the treatment-planning system and protocol. Practical check: verify the revised field dimensions and associated calculated dose/MU in the plan, then confirm the record-and-verify parameters match the approved plan before treatment.