An experimental investigation into the radiation field offset of a dynamic multileaf collimator

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In this study we investigate the characteristics of a rounded leaf end multileaf collimator (MLC) that is used for delivering intensity-modulated radiotherapy (IMRT) with a Varian linear accelerator. The rounded leaf end MLC design results in an offset between the radiation field edge (the physical leaf position) and the light field (the geometric leaf position). We call this the radiation field offset (RFO). The leaf position is calibrated to the leaf tip at the mid-leaf plane. There is an additional offset between the geometric leaf position and the projected leaf tip position that varies as a function of distance from the collimator central axis due to the MLC geometry. We call this the leaf position offset (LPO). There is a lack of consistency in the interpretation and implementation of the RFO and the LPO in the literature. We investigated the RFO and the LPO on Varian's 600 C/D and 21 EX linear accelerators. We used a combination of film and ion chamber measurements of static, segmental MLC (SMLC) and dynamic MLC (DMLC) fields to quantify the leaf offsets across the range of leaf positions. We were able to improve the dosimetry at large off-axis positions with minor adjustments to the vendor's LPO file. The RFO was determined to within 0.1 mm accuracy at the collimator central axis. The measured RFO value depends on whether the method is based on the radiation field edge position or on an integral dose measurement. The integral dose method results in an RFO that is approximately 0.2 mm greater than the radiation field edge method. The difference is due to the MLC penumbra shape. We propose a methodology for measuring and implementing MLC leaf offsets that is suitable for both SMLC and DMLC IMRT. In addition, we propose some definitions that more clearly describe the MLC leaf position for accurate IMRT dosimetry.