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.