Purpose: Brachytherapy procedure may result in acute tissue reactions like edema, causing deviations between planned and measured doses. The rationale for in vivo dosimetry in interstitial brachytherapy is to assess the accuracy of the delivered dose in comparison with the dose calculated by the treatment planning system (TPS). Material and methods: One single computer tomography (CT) dataset was used for brachytherapy planning, taken within 24 hours after implantation. In vivo interstitial measurements with micro-MOSFET-detectors (metal oxide semiconductor field effect transistor) were performed in 12 patients with different anatomic locations of cancers, including thorax-wall, head and neck, breast, and different types of implantations (monoplanar, loops, and multiplanar). Results: Measured values for the thorax-wall tumor patient showed a good agreement with the calculated data, with average deviation of –2.7% in 8 mm distance to the closest dwell position of the source. The deviation of the measured dose value of the head and neck patient was +55.6% in the first fraction and +8.5% in the last fraction. In the ten breast cancer patients, measured doses depended on the proximity of the detector to the irradiated volume PTV. Conclusions: The deviations between planned and measured dose values were markedly influenced by the proximity of the detector to the PTV because where the edema exerts, the greatest influence on the tube applicator geometry. The positioning of the patient during irradiation must correspond to the positioning in the planning CT. Further studies are needed to investigate the role of in vivo dosimetry during interstitial brachytherapy as a routine procedure.