Disclosed herein is a marker-flange for use with a brachytherapy applicator, having a flange body with a first face, a second face, and a hollow chamber positioned between the first and second faces. A cavity extending through the flange body is dimensioned to receive a tandem in a brachytherapy applicator in a press-fit connection, so as to affix the flange-marker to an outside surface of the tandem. The hollow chamber includes an MR imaging responsive marker agent that provides strong signal intensities and a high geometric accuracy in MR imaging.