Radiation brings hope and fear: hope of fighting cancer with radiotherapy and fear of sickness induced by radiation dose. Recent years have seen many advances in novel radiotherapy methods, including the rapidly growing field of particle therapy. I will focus mostly on the proton therapy method, available in Poland since 2011. Computer modelling plays an important role in modelling of radiation interaction with matter, leading to improved treatment outcomes and better understanding of the therapeutic process at many levels. Such modelling is required for the process of therapy planning, in which radiation beams are shaped to obtain the best coverage of the tumor region. In this talk I will present several areas in which simulations of particle interactions with matter lead to new ideas and provide insight into cancer treatment with radiotherapy.