Calibration and quality assurance for rounded leaf-end MLC systems

Multileaf collimator MLC) systems are available on most commercial linear a ccelerators, and many of these MLC systems utilize a design with rounded le af ends and linear motion of the leaves. In this kind of system, the agreem ent between the digital MLC position readouts and the light field or radiat ion field edges must be achieved with software, since the leaves do not mov e in a focused motion like that used for most collimator jaw systems. In th is work we address a number of the calibration and quality assurance issues associated with the acceptance, commissioning. and routine clinical use of this type of MLC system. These issues are particularly important for MLCs used for various types of intensity modulated radiation therapy (IMRT) and small, conformal fields. For rounded leaf end MLCs, it is generally not pos sible to make both the light and radiation field edges agree with the digit al readout, so differences between the two kinds of calibrations are illust rated in this work using one vendor's MLC system. It is increasingly critic al that the MLC leaf calibration be very consistent with the radiation fiel d edges, so in this work a methodology for performing accurate radiation fi eld size calibration is discussed. A system external to the vendor's MLC co ntrol system is used to correct or handle limitations in the MLC control sy stem. When such a system of corrections is utilized, it is found that the M LC radiation field size can be defined with an accuracy of approximately 0. 3 mm, much more accurate than most vendor's specifications for MLC accuracy . Quality assurance testing for such a calibration correction system is als o demonstrated. (C) 2001 American Association of Physicists in Medicine.