Radiation therapy by photons is commonly utilized for cancer treatment. Ionising radiation is used to destroy tumour cells by damaging their DNA, making it impossible for these cells to continue to grow and divide. The accuracy of the dose delivery is of great importance. Larger dose causes significant complications in the normal tissue, whereas reduced dose decreases the probability of tumour control. Therefore the treatment is carefully planned. Tumours are diagnosed and localized by 3D imaging techniques and the irradiation of the patient is simulated by advanced software referred as a treatment planning system (TPS). Different simplifications and assumptions are used in the TPS algorithms in order to keep balance between the requirements for fast calculations and for sufficient accuracy of the result. Validation of the results from TPS is highly recommended, particularly in situations of inhomogeneous media and interfaces due to lack of electron equilibrium and incomplete scatter conditions. Conditions of electron disequilibrium are typical for complex treatment techniques such as volumetric modulated arc therapy (VMAT). Thus, theoretical validation of the planned dose distribution and experimental validation of treatment deliverability are important part of pre-treatment patient specific quality assurance. The project objective is to provide pre-treatrment and in-vivo validation av radiation treatment by applying the Monte Carlo (MC) method. Clinical dose distributions in patient geometry will be calculated by the MC method and compared with planned distributions using Gamma index or NDD evaluation. The response of cylindrical ionization chambers to high-energy photon beams will be simulated in case of pre-treatment phantom measurements. MC method will be applied to develop and implement in vivo dosimetry system using Electron Portal Image Device (EPID). EPID response to irradiation transmitted through the patient and its sensitivity to patient mispositioning and anatomy changes will be investigated.