Impact of Multileaf Collimator Configuration Parameters on the Dosimetric Accuracy of 6-Mv Intensity-Modulated Radiation Therapy Treatment Plans

Abstract

Purpose: To improve the dosimetric accuracy of intensity modulated radiation therapy (IMRT) dose distributions as calculated by the treatment planning system (TPS) by optimizing the parameters that govern multileaf collimator (MLC) transmission and rounded leaf offset. Methods: The MLC leaf transmission was optimized based on measurements made with ionization chambers and radiographic film. The rounded leaf offset table was optimized by measuring the radiation field edge as a function of leaf bank position with an ionization chamber in a water scanning tank and comparing the location to TPS equivalent dose calculations. Optimizations were validated by performing IMRT quality assurance (QA) tests on 19 gantry-static IMRT plans. Planar dose measurements were performed with film and a planar diode array and compared to TPS calculated dose distributions with default and optimized parameters. Results: Based on measurements, the leaf transmission factor was changed from a default value of 0.001 to 0.005. This optimization resulted in a statistically significant worsening of IMRT QA gamma index passing rate, because the currently used model is already slightly higher than the measured data originally used to commission the machine. The rounded leaf offset table had little room for improvement, with the average difference between the default and optimized offset values being -0.2 ± 0.7 mm. This reflects the excellent leaf position calibration protocol of physics staff. Conclusion: The hypothesis that TPS dosimetric accuracy of IMRT fields could be improved by optimizing the rounded leaf offset table and MLC transmission parameters was not supported by the results of this work.