The invention relates to a system 1 for planning a treatment of a subject 2. A treatment planning device 39 determines a treatment position of a treatment device like a needle or catheter which comprises an energy delivery element not completely encircling the treatment device. The determined treatment position is a position in a four-dimensional space being representable by three Cartesian coordinates and an angular coordinate defining the angular orientation of the treatment device with respect to a rotation around its longitudinal axis. The treatment device is then arranged in accordance with the treatment position. Since not only the three-dimensional position of the treatment device is determined, but also a rotational position of the treatment device with respect to a rotation of the treatment device around its longitudinal axis, the energy delivery can be very accurately planned, leading to a reduction of unwanted therapy side effects.