Monte Carlo based treatment planning for modulated electron beamradiation therapy

Abstract

A Monte Carlo based treatment planning system for modulated electronradiation therapy (MERT) is presented. This new variation ofintensity modulated radiation therapy (IMRT) utilizes an electron multileafcollimator (eMLC) to deliver non-uniform intensity maps at several electron energies.In this way, conformal dose distributions are delivered to irregulartargets located a few centimetres below the surface while sparingdeeper-lying normal anatomy. Planning for MERT begins with Monte Carlo generationof electron beamlets. Electrons are transported with proper in-airscattering and the dose is tallied in the phantom for each beamlet.An optimized beamlet plan may be calculated using inverse-planning methods.Step-and-shoot leaf sequences are generated for the intensity maps anddose distributions recalculated using Monte Carlo simulations. Here,scatter and leakage from the leaves are properly accounted for bytransporting electrons through the eMLC geometry. The weights for thesegments of the plan are re-optimized with the leaf positions fixed andbremsstrahlung leakage and electron scatter doses included. Thisoptimization gives the final optimized plan. It is shown that asignificant portion of the calculation time is spent transportingparticles in the leaves. However, this is necessary since optimizingsegment weights based on a model in which leaf transport is ignoredresults in an improperly optimized plan with overdosing of target andcritical structures. A method of rapidly calculating the bremsstrahlungcontribution is presented and shown to be an efficient solution to thisproblem. A homogeneous model target and a 2D breast plan are presented.The potential use of this tool in clinical planning is discussed.