AbstractID: 7832 Title: A multi-leaf collimator model for accurate IMRT Monte Carlo dose calculation Monte Carlo (MC) algorithms are recognized as the most accurate for patient dose assessment. For IMRT delivered with dynamic multi-leaf collimators (dMLCs) accurate dose calculation, even with MC, is challenging. Accurate IMRT MC dose calculations require inclusion of the moving MLC in the MC simulation. Due to its complex geometry, full transport through the MLC can be time consuming. The aim of this work was to develop an MLC model for clinical MC IMRT dose computations. The basis of the MC MLC model is that the complex MLC geometry can be separated into simple geometrical regions, each of which readily lends itself to simplified radiation transport. The amount of attenuation material an individual particle encounters while traversing the entire MLC is determined by adding the individual amounts from each of the simplified geometric regions. We have tested the MLC model by comparing it with measurements and MC simulations that incorporate the full MLC geometry for extreme limiting cases of MLC transport, for uniform sliding windows, and for patient IMRT cases. We have found that the MC MLC model predicts the leaf-edge tongue-and-groove dose perturbation effect within ±2% or 2 mm. Similarly, the model correctly predicts the beam hardening produced by the MLC and the entrance dose behind a closed MLC. Tests with uniform sweeping gaps and IMRT patient test cases show that the MLC model reproduces measurement results within ±2% or 2 mm. This model is used clinically for MC dosimetric verification of every IMRT case treated at our institution.