The invention relates to a HDR brachytherapy system comprising an ultrasound sensor for being arranged at the location of a brachytherapy catheter (12), wherein the ultrasound sensor is adapted to generate an ultrasound signal based on ultrasound radiation, which has been sent by an ultrasound imaging device preferentially comprising a TRUS probe (40) and which has been received by the ultrasound sensor. The position of the ultrasound sensor is determined based on the generated ultrasound signal, and based on this position of the ultrasound sensor the pose and shape of the brachytherapy catheter and/or the position of a HDR radiation source are determined. This allows for a very accurate determination of the pose and shape of the brachytherapy catheter and/or of the position of the HDR radiation source, which in turn can lead to an improved HDR brachytherapy.
