The construction and first proton beam tests of a demonstrator dedicated to the beam ballistic control in hadrontherapy cancer treatments are described. This cost-effective demonstrator, called large area pixelized detector, is a PET-like detector used for in-beam ballistic control. It was built to test the feasibility of monitoring in real time, during irradiation, the ion range in the patient through the measurement of the beam-induced β + activity distribution. Achieving this goal necessitates to overcome several challenges. One of them is the rejection of the beam-induced background. Another one is the definition of fast event selection and reconstruction techniques so that real time monitoring is possible. Strategies employed to tackle these problems are presented and tested with the 65 MeV Medicyc proton beam of the cancer treatment center in Nice, France. In particular, an original fast reconstruction technique is presented. First performances obtained during irradiation of polymethyl methacrylate targets are described.