Currently, radiotherapy with electrons and photons beams is the most common approach in health care facilities; it allows widespread use of technology. Only a fast and accurate verification of the dose characteristics of the beams provides a safe and effective use of the methodology. We have designed, built and tested a new instrument based on a layer of square scintillating fibers of 0.5 mm thickness that can run a scan of the beam. The system provides the dosage profiles, related to a plane orthogonal to the beam axis at a fixed depth. The device was tested with a photon beam of 6 MV delivered by a linear accelerator Varian Clinac DHX 2100. The data collected were processed for the tomographic reconstruction of the dose by means of the FBP (filtered back projection) algorithm with different filters. An iterative reconstruction algorithm was also tested. In this paper, the results are presented and suggestions for future developments are made.