A Monte Carlo investigation of fluence profiles collimated by an electron specific MLC during beam delivery for modulated electron radiation therapy.

Modulated electron radiation therapy (MERT) is able to deliver conformal dose to shallow tumors while significantly reducing dose to distal structures and surrounding tissues. An electron specific multileaf collimator (eMLC) has been proposed and constructed as an effective means of delivering electron beams for MERT. The aim of this work is to apply the Monte Carlo method to investigate the fluence profiles collimated by the eMLC in order to achieve accurate beam delivery for MERT. In this work, the EGS4/BEAM code was used to simulate the eMLC collimated electron beams of 6-20 MeV generated from a Varian Clinac 2100C linear accelerator. An attempt was made to describe the fluence profiles with an analytic Sigmoid function. The function parameters were determined by the fittings of the Monte Carlo simulated fluence profiles. How the function parameters depend on the eMLC aperture size, the off-axis location, and the electron beam energy has been investigated. It has been found that the eMLC collimated fluence profiles are dependent on beam energy, while almost independent of leaf location or dimension of MLC aperture. There is little difference in the fluence profiles collimated by the leaf side and the leaf end for the straight-edged leaves. It is possible that these energy-dependent Sigmoid functions can serve as operators to account for the energy dependence of the eMLC collimated fluence profiles. These operators can be incorporated into the inverse planning algorithm to derive desired dose distributions using a set of electron beams of variable energy and field size suitable for delivery by the eMLC.