The work was divided into three phases. The first was simulating and verifying ELEKTA SL18 Linear accelerator for electron beams using EGSnrc Monte Carlo Codes (BEAMnrc, DOSXYZnrc and BEAMDP) for different energies, field sizes and gantry angles. The simulated beams were then used for postmastectomy radiotherapy using transported patient CT scans in DICOM format, in the second phase. The three- dimensional depth dose files produced by DOSXYZnrc were transfered to XSTING, a home build software that was previously designed for stertactic radiotherapy and then modified for electron and photon radiotherapy, to generate dose-volume-histograms (DVHs). The generated DVHs were used to evaluate the risk to the lung and the heart. In the third phase a home-built dose reponse evaluation tool (DORES) was used to estimate the risk of radiation induced pneumonitis and lung fibrosis. Single electron beams were proved to induce low risks to the heart and lung, but had the limitation of poor target coverage. Multiple fields were recommended for frther investigation.