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 mul tiple of multileaf collimator (MLC) subfields. A subfield with a small leaf -to-leaf opening is highly sensitive to a leaf-positional error. We introdu ce a method of identifying and rejecting IMRT plans that are highly sensiti ve to a systematic MLC gap error (sensitivity to possible random leaf-posit ional 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 rou nded end MLC, radiation field offset (RFO). In IMRT planning system, using an incorrect value of RFO introduces a systematic error Delta RFO that resu lts in all leaf-to-leaf gaps that are either too large or too small by (2 . Delta RFO), whereas assuming that CMO is zero introduces systematic error Delta CMO that results in all gaps that are too large by Delta CMO=CMO. We introduce a concept of the average leaf pair Opening (ALPO) that can be cal culated from a dynamic MLC delivery file. We derive an analytic formula for a fractional average fluence error resulting from a systematic gap error o f Ax and show that it is inversely proportional to ALPO; explicitly it is e qual to Deltax/(ALPO+ 2 . RFO + epsilon), in which epsilon is generally of the order of 1 mm and Deltax = 2 . Delta RFO + CMO. This analytic relations hip is verified with independent numerical calculations. (C) 2001 American Association of Physicists in Medicine.