A brachytherapy after loader device (100), comprises at least one transmit wire (102) that is suitable for being inserted and driven in an advance or retract motion in an external first delivery channel (103) and for delivering drive energy to a test field source (104), which is arranged at a distal end region of the transmit wire, for generating a magnetic test field； at least one receive wire (106) that is suitable for being inserted and driven in an advance or retract motion in an external second delivery channel (105) for a measurement of the magnetic test field, the receive wire having a transducer (108) that is configured to detect magnetic-field changes in the magnetic test field； and a wire driving unit (110) which is configured to controllably advance or retract the transmit wire and the receive wire in response to a corresponding test drive control signal (112).
