The in-beam PET is a novel PET application to image the beta+ activity induced in biological tissues by hadronic therapeutic beams. Thanks to the correlation existing between beam-delivered dose profiles and beam-induced activity profiles, in vivo information about the effective ion paths can be extracted from the in-beam pet image. in situ measurements, immediately after patient irradiation, are recommended in order to exploit the maximum statistics, by also detecting the contribution provided by the very short lived isotopes, e.g. 15O. A compact, dedicated tomograph should then be developed for such an application, so as to be used in the treatment room. We developed a small PET prototype in order to demonstrate the feasibility of such a technique for the monitoring of proton therapy of ocular tumors at the CATANA facility (Catania, Italy). The prototype consists of two planar heads with an active area of about 5 cm times 5 cm. Each head is made up of a square position sensitive photomultiplier (Hamamatsu H8500) coupled to a matrix of the same size of LYSO scintillating crystals (2 mm times 2 mm times 18 mm pixel dimensions). Dedicated, compact electronic boards are used for the signal multiplexing, amplification and digitization. The distance between the pair can be varied from 10 cm up to a maximum of about 20 cm. The validation of the prototype was performed on plastic phantoms using 62 MeV protons at the CATANA beam line. Different dose distributions were delivered and a good correlation between the distal fall-off of the activity profiles and of the dose profiles was found, i.e., better than 2 mm along the beam direction.