Approximately 70% of men with newly diagnosed prostate cancer feature organ-confined disease. Conventional treatment options for prostate-confined cancer include radical prostatectomy, external beam radiation therapy, brachytherapy and active surveillance. All currently available treatments have comparable toxicity risk and high social and economic impact, hence the best treatment option has not been defined yet. In the radiation oncology field the gold standard for prostate cancer management is 7-8 weeks intensity modulated radiotherapy (IMRT), which can be delivered with the latest generation of accelerators providing modulated arc delivery (eg. RapidArc™, Varian) or with newly developed machines such as the Vero system (BrainLab AG - Mitsubishi Heavy Industries Ltd). The present research project aims to improve the current treatment for prostate confined tumor, evaluating the best strategy to deliver a very-short hypofractionated radiotherapy scheme. In the first part of this AIRC grant an in-silica study will be performed on 10 test prostate cancer patients, with comparative competitive treatment plans with the state of the art treatment options for prostate cancer: the Vero system, the RapidArc system, the Cyberknife (all available at the European Institute of Oncology IEO, Milan, Italy), and protontherapy (available at the Centre of Adrotherapy, in Pavia, Italy). This dosimetric comparison will be performed in order to define the technique that provides the maximum target coverage with the minimum dose to the surrounding organs at risk (OARs) to be used to perform the clinical trial that will follow within this AIRC grant. After performing the in-silica study, 65 newly diagnosed prostate cancer patients with 2011 National Comprehensive Cancer Network (NCCN) risk category ranging from very low to intermediate but not undergoing hormonal treatment, will be consecutively enrolled in a prospective phase II trial. The patients will be treated at the Division of Radiation Oncology of the IEO, with the dose delivery system that will result most fit at the in-silica study. Simultaneous Integrated Boost (SIB) technique will be applied to deliver a total dose of 36.25 Gy in 5 fractions (over 10 days) to the whole prostate (7.25 Gy/fraction) and 37.5 Gy to the dominant intraprostatic lesion DIL (7.5 Gy/fraction), profiting of the high sensibility of prostate cancer to high dose/fraction. Several strategies will be applied in order to reduce the dose to the surrounding OARs. In order to define the localization of the DIL, multiparametic magnetic resonance imaging (MRI) of the pelvis will be performed (with spectroscopy, diffusion weighted and perfusion acquisitions), and fused with computed tomography (CT) scan. In-room image-guidance will be applied at each treatment section, in order to minimize uncertainties in intra and inter-fraction prostate localization. At the end of the radiotherapy course, each patient will be followed-up, in order to assess the treatment safety and effectiveness in terms of early and late toxicity, and tumor control. A sampling from the prostate tumor tissue for biological study will be taken for microarray analysis. The expression of specific markers of radiosensitivity and radioresistance will be investigated.