Method of identifying dynamic multileaf collimator irradiation that is highly sensitive to a systematic MLC calibration error

In intensity modulated radiotherapy (IMRT), radiation is delivered in a multiple of multileaf collimator (MLC) subfields. A subfield with a small leaf‐to‐leaf opening is highly sensitive to a leaf‐ positional error. We introduce a method of identifying and rejecting IMRT plans that are highly sensitive to a systematic MLC gap error (sensitivity to possible random leaf‐positional errors is not addressed here). There are two sources of a systematic MLC gap error: centerline mechanical offset (CMO) and, in the case of a rounded end MLC, radiation field offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error ΔRFO that results in all leaf‐to‐leaf gaps that are either too large or too small by (2⋅ΔRFO), whereas assuming that CMO is zero introduces systematic error ΔCMO that results in all gaps that are too large by ΔCMO=CMO. We introduce a concept of the average leaf pair Opening (ALPO) that can be calculated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error of Δ x and show that it is inversely proportional to ALPO; explicitly it is equal to Δ x / ( ALPO + 2 ⋅ RFO + ε ) , in which ε is generally of the order of 1 mm and Δ x = 2 ⋅ Δ RFO + CMO . This analytic relationship is verified with independent numerical calculations.