[en] A system for in vivo tracking of 1 Ci ¹⁹²Ir source during brachytherapy treatment has been built using high resistivity silicon pad detectors as image sensors and knife-edge lead pinholes as collimators. The sensors consist of 256 pads arranged in 32 x8 grid with pad size 1.4x1.4mm² and 1 mm thickness. The sensors have two metal layers, enabling connection of readout electronics (VATAGP3₁ chips) at the edge of the detector. With source self-images obtained from a dual-pinhole system, location of the source can be reconstructed in three dimensions in real time, allowing on-line detection of deviations from planned treatment. The system was tested with 1 Ci ¹⁹²Ir clinical source in air and plexi-glass phantom. The movements of the source could be tracked in a field of view of approximately 20x20x20cm³ with absolute precision of about 5 mm. Positions of the source, relative to the first measured source position, could be mapped with precision of around 3 mm.