The present work introduces a continuous model for the response of primary brain tumour growth to radiation therapy. The progression of the tumour is based on a partial differential equation which describes the proliferation of tumour cells as well as the anisotropic diffusion into surrounding tissue. This reaction-diffusion equation is extended by an additional term modelling the effect of radiation therapy. For the determination of the radiation effect, the linear-quadratic radiobiological model is used. To account for the influence of different biological mechanisms on the treatment response, a variation of radiation sensitivity inside the tumour tissue is incorporated. Additionally, the dynamics of necrotic cells and the effect of radiation therapy on healthy tissue are considered. The resulting model allows for simulating different treatment schedules. The qualitative analysis of first results displays a plausible description of radiation effects on tumour growth and healthy tissue. A quantitative evaluation is challenging as patient individual parameters have to be estimated and, thus, remains as ongoing research.