A comparison of Monte-Carlo simulated and measured dose distributions in oblique electron beams

Oblique incidence of electron beams is a rare clinical application which can be used e.g. at the irradiation of the internal mammary lymph nodes in the case of mamma carcinoma. 3-D treatment planning systems are accepted standard for photon beams but not for electron beams. The in for measuring, implementing and testing basic data seems many customers not to be worthwhile when considering the well known inaccuracies of conventional algorithms. From Monte-Carlo (MC) based algorithms however higher accuracies can be expected. It was the aim of this paper to test the MC algorithm of the treatment planning system Oncentra Treatment Planning (OTP, Theranostic) in the application for oblique incidence of electron beams. Measured and simulated values are compared for electron energies from 4 MeV up to 18 Me V. For standard electron applicators, even at extended source-surface distances, satisfying results are achieved. However, the agreement gets lost with increasing oblique incidence, especially for small individual and asymmetrical cut outs. We therefore still use measured values instead of simulated dose distributions.