Due to the features of modern radiotherapy techniques, namely Intensity Modulated Radiation Therapy (IMRT) and Stereotactic Treatments with photon fields, where high spatial dose gradient are often present, detectors to be employed for 2D dose verifications have to satisfy very narrow requirements. In particular, they have to exhibit high spatial resolution. In the framework of the European Integrated project MAESTRO, a dosimetric detector adequate for 2D pre-treatment dose verifications was developed. It is a modular detector based on a monolithic silicon segmented sensor, with a n-type implantation on an epitaxial p-type layer. The sensor is composed of 21×21 pixels with 2×2 mm 2 area and 3 mm center-to-center distance. In this paper we report the first preclinical dosimetric characterization of the detector. The sensor was irradiated with 6, 10, 25 MV photon beams from a LINAC. The studied parameters were repeatability of a single channel, reproducibility between different matrix elements, response linearity versus absorbed dose and dose rate dependence of sensitivity. Some preliminary tests on energy dependence were also performed. The obtained results are promising since almost all the channels have performances within the project specifications (repeatability <0.5%, reproducibility <1%, deviation from linearity <1%, dose rate dependence <1%).