An applicator device (100, 200, 300) for use in interventional brachytherapy, wherein the applicator device (100, 200, 300) is configured to be inserted in, or near a region of interest within a living body, and the applicator device is defining a lumen (101, 201, 301) for receiving a radiation source (103, 203, 303), characterized in that the applicator device (100, 200, 300) comprises one or more sensor elements (102, 202, 302) each configured to generate an output signal indicative of whether or not the radiation source (103, 203, 303) is at a predetermined position in the lumen (101, 201, 301).