Metal Oxide Semiconductor (MOS) device structures can be used to measure ionizing radiation through the mechanism of hole trapping in the oxide layer leading to changing of electrical characteristic of the device. They are a new type of direct reading semiconductor dosimeters. Due to their extremely small physical size, ability to permanently store the accumulated dose, dose-rate independence and their ease of use make them very promising for in vivo dosimetry. They are attractive for dosimetry in small radiation fields used in modern radiation oncology modalities, as conformal radiotherapy, IMRT, stereotactic radiotherapy/radiosurgery and proton therapy. Preliminary results on the use of commercial MOSFET dosimeters (TN-502RD, Thomson & Nielsen Electronics Ltd, Canada) irradiated on therapeutic 62 MeV proton beams are presented. Linearity with absorbed dose, sensibility and energy dependence were investigated. Moreover, the possibility to use of MOSFET dosimeters in order to measure the Output Factors (OF) for very small irradiation fields was verified. The comparison of OF obtained using MOSFETs and other dosimetry systems is reported.