High energy electron beams in intensity modulated radiation therapy

Abstract

The aims of the thesis were the development of the phase space evolution (PSE) model for electron beam treatment planning, investigation of techniques to improve electron beam dose distributions, and investigation of the added value of high energy (⩽50 MeV) electron beams in intensity modulated radiotherapy. The PSE model is capable of calculating dose distributions by taking into account density variations in full three dimensions and is accurate and fast compared to Fermi–Eyges based pencil beam models or Monte Carlo models. A relatively simple model that requires only measurements as input is adequate to describe the initial electron beam. The PSE model can also be used for the calculation of pencil beam kernels for optimization of intensity modulated electron beam treatments. Techniques, such as intensity modulation, energy modulation, and mixing with photons are suggested to improve dose distributions resulting from electron beam irradiations from a single direction. It is shown that, by the combination of intensity modulated electron and photon beams from one direction, dose distributions can have steep dose falloffs similar to unmodulated electron beams, and narrow penumbras similar to photon beams. The added value of high energy electron beams was investigated in a comparative treatment planning study using optimized intensity modulated electron and photon beams. For the studied clinical cases use of intensity modulated photon beams gives significant improvements over conventional treatments. Inclusion of electron beams could only marginally improve the expected treatment outcome, although dose volume histograms of organs at risk show improvements, especially at lower dose levels (e.g., <40 Gy) in the treatments with electrons.