The invention relates to an assembly for performing brachytherapy treatment of tumour tissue in a human or animal body comprising an intracavitary component and at least one guiding unit, said at least one guiding unit exhibiting a longitudinal axis and being connectable to said intracavitary component, and wherein said at least one guiding unit is provided with coupling means for coupling at least one interstitial needle-assembly for delivering treatment to said tumour tissue.The assembly has coupling/uncoupling means arranged to allow a displacement of said at least one interstitial needle-assembly in a direction transverse to said longitudinal axis and to prevent a displacement of said at least one interstitial needle-assembly in a direction parallel to or a pre-defined angle to said longitudinal axis. This coupling principle allows for the preparation of the guiding unit before the patient is being hospitalized and positioned in the treatment room and will prevent the unwanted uncoupling of the interstitial needle-assembly during the actual treatment being performed, further reducing the risk of adversely affected treatment parameters (location and duration of the radiation being administered to the patient).
