AbstractID: 9121 Title: Development of a Monte Carlo Model for Dose Calculations in
IMRT
Intensity modulated radiotherapy provides improved target coverage and reduced doses to surrounding normal tissues compared with
conformal radiotherapy. However, quality assurance is more challenging for the complex fluence maps used in IMRT treatments, and
direct measurements can be labor-intensive. A Monte Carlo based phase space model has been developed which produces accurate
dose calculations for IMRT using the Novalis linear accelerator. The basis for the model is the MCNP4c code, which accounts for the
lack of lateral electronic equilibrium present in the small fields used in IMRT. The open beam fluence is calculated by simulating the
components of the linear accelerator treatment head above the multileaf collimator. This fluence map is then adjusted to match the
physical beam using an intensity grid, which incorporates a detailed model of the multileaf collimator. The intensity grid accounts for
the shaped leaf tip geometry and the beam divergence that influence the dose at the edge of the open leaves. It incorporates the
transmission through the leaves, the leakage between them, and the tongue-and-groove design, which affect the dose under the leaves.
The variation in beam energy across the field is also considered. Our model provides accurate dose calculations of arbitrary IMRT
treatment plans in homogeneous and heterogeneous media and the ability to simulate leaf sequencing effects, using both segmented
and dynamic sequencing.