Development and commissioning of a multileaf collimator model in Monte Carlo dose calculations for intensity-modulated radiation therapy

A multileaf collimator (MLC) model, “MATMLC,” was developed to simulate MLCs for Monte Carlo (MC) dose calculations of intensity-modulated radiation therapy (IMRT). This model describes MLCs using matrices of regions, each of which can be independently defined for its material and geometry, allowing flexibility in simulating MLCs from various manufacturers. The free parameters relevant to the dose calculations with this MLC model included MLC leaf density, interleaf air gap, and leaf geometry. To commission the MLC model and its free parameters for the Varian Millennium MLC-120 (Varian Oncology Systems, Palo Alto, CA), we used the following leaf patterns: (1) MLC-blocked fields to test the effects of leaf transmission and leakage; (2) picket-fence fields to test the effects of the interleaf air gap and tongue-groove design; and (3) abutting-gap fields to test the effects of rounded leaf ends. Transmission ratios and intensity maps for these leaf patterns were calculated with various sets of modeling parameters to determine their dosimetric effects, sensitivities, and their optimal combinations to give the closest agreement with measured results. Upon commissioning the MLC model, we computed dose distributions for clinical IMRT plans using the MC system and verified the results with those from ion chamber and thermoluminescent dosimeter measurements in water phantoms and anthropomorphic phantoms. This study showed that the MLC transmission ratios were strongly dependent on both leaf density and the interleaf air gap. The effect of interleaf air gap and tongue-groove geometry can be determined most effectively through fence-type MLC patterns. Using the commissioned MLC model, we found that the calculated dose from the MC system agreed with the measured data within clinically acceptable criteria from low- to high-dose regions, showing that the model is acceptable for clinical applications.