A light field‐based method to adjust rounded leaf end MLC position for split shape dose calculation correction in a radiation therapy treatment planning system

We present an analytical and experimental study of split shape dose calculation correction by adjusting the position of the round leaf end position in an intensity‐modulated radiation therapy treatment planning system. The precise light field edge position ([Formula: see text]) was derived from 50% of the central axis dose created by nominal light field using geometry and mathematical methods. Leaf position ([Formula: see text]), defined in the treatment planning system for monitor unit calculation, could be derived from [Formula: see text]. Offset (correction) could be obtained by the position corresponding to 50% of the central axis dose minus the [Formula: see text] position. For SSD from 90 cm to 120 cm at 6 MV and 10 MV, the 50% dose position was located outside of [Formula: see text] in the MLC leaf position range of [Formula: see text] to [Formula: see text] , where the offset correction positively increased, whereas the offset correction negatively increased when the MLC leaf position was in the range of [Formula: see text] to [Formula: see text] and [Formula: see text] to [Formula: see text] when the 50% position was located inside [Formula: see text]. The monitor unit calculation could provide underdosage or overdosage of 7.5% per mm without offset correction. Calibration could be performed at a certain SSD to fit all SSD offset corrections. With careful measurement and an accurate offset correction, it is possible to achieve the dose calculation with 0.5% error for the adjusted MLC leaf edge location in the treatment planning system. PACS number: 87.53.Tf, 87.55.x, 87.55.D, 87.55.dk