Characterization of electron beams for modulated electron beam radiotherapy

A set of clinical electron beams have been characterized in anticipation of their use in modulated electron radiotherapy (MERT). Using the EGS4/BEAM and DOSXYZ simulation codes, a Monte Carlo model of a Varian Clinac 2100C was generated for electron beams of nominal energies of 6, 9, 12, 16, and 20 MeV. Dose calculations using the Monte Carlo generated phase space data demonstrate profile and depth-dose agreement with measured data to within 2% of the maximum dose values. Simulations were then performed using a proposed electron multileaf collimator to examine beam characteristics relevant to MERT. In particular, abutting fields have been constructed and the presence of hot and cold spots due to penumbral mismatches is shown to be significant. The effect of field setup errors is also considered and it is shown that small setup uncertainties of 3 mm may lead to significant hot and cold spots of up to 25%. Furthermore, electron beamlets were shown to differ in penumbras and symmetry based on where they were located in the field. Intensity modulated electron fields were then constructed, demonstrating the ability to create fields with tilted dose profiles using a sequential abutting field method and a dynamic delivery method.