Purpose: Due to the increase of treatment complexity related to the improvement of optimization techniques and new model-based dose calculation algorithms, high dose rate (HDR) brachytherapy with Ir-192 would benefit from in vivo dosimetry to ensure correct dose delivery. Metal oxide semiconductor field effect transistors (MOSFET) have been proposed as in vivo dosimeters due to their small sensitive volume and direct readout. The prototype RADPOS system (Best Medical Canada) consists of a MOSFET dosimeter physically coupled to a position-sensing probe, which deduces its 3D position in static magnetic fields generated by an emitter. The purpose of this study was to assess the accuracy of the positioning device and the feasibility of Ir-192 in vivo dosimetry with the RADPOS system.Methods: Using a PMMA phantom, the positioning accuracy of the RADPOS system was assessed by comparing position readouts with the known position of the detector along the X, Y and Z axes. RADPOS dose measurements were performed at several distances from a Nucletron Ir-192 source in a PMMA phantom. The photon spectrum at each distance from the Ir-192 source was calculated using Monte Carlo (MC) simulations to determine the energy response of the micro-MOSFET used in the RADPOS system. With this knowledge the RADPOS system can be used at arbitrary positions in a PMMA phantom in order to evaluate the delivered dose for realistic plans. Results: Positioning accuracy was found to be within 1mm in the 1–10 cm range from the source, meeting the requirements for in vivo dosimetry. Similar results are obtained for positions along the 3 axes. Calibration factors were determined at different distances from the source to evaluate the energy dependence of the micro-MOSFET. Conclusions: Combining accurate 3D positioning data and dose measurements, the RADPOS system shows promise as an in vivo dosimetry system for HDR brachytherapy.