A high-dose rate brachytherapy system for performing a high-dose rate brachytherapy, the high-dose rate brachytherapy system comprising: - a brachytherapy catheter (12; 112; 212) to be inserted into or close to a target region inside a living being, - an elongated introduction element (13; 113; 213) with a radiation source for applying radiation emitted by the radiation source to the target region, wherein the brachytherapy catheter (12; 112; 212) and the introduction element (13; 113; 213) are adapted to allow the introduction element (13; 113; 213) to be introduced into the brachytherapy catheter (12; 112; 212), the system characterised by further comprising: - an ultrasound imaging device (40, 42) for generating an ultrasound image of the inside of the living being by sending ultrasound radiation into the inside of the living being and by measuring ultrasound radiation reflected by the inside of the living being, - an ultrasound sensor for being arranged at the location of the brachytherapy catheter (12; 112; 212) within the living being, wherein the ultrasound sensor is adapted to generate an ultrasound signal based on ultrasound radiation sent by the ultrasound imaging device (40, 42) and received by the ultrasound sensor, - a position determining unit for determining the position of the ultrasound sensor based on the generated ultrasound signal and for determining the pose and shape of the brachytherapy catheter (12; 112; 212) within the living being and/or the position of the radiation source, when the introduction element (13; 113; 213) has been introduced into the brachytherapy catheter (12; 112; 212), based on the determined position of the ultrasound sensor. - a brachytherapy catheter (12; 112; 212) to be inserted into or close to a target region inside a living being, - an elongated introduction element (13; 113; 213) with a radiation source for applying radiation emitted by the radiation source to the target region, wherein the brachytherapy catheter (12; 112; 212) and the introduction element (13; 113; 213) are adapted to allow the introduction element (13; 113; 213) to be introduced into the brachytherapy catheter (12; 112; 212), the system characterised by further comprising: - an ultrasound imaging device (40, 42) for generating an ultrasound image of the inside of the living being by sending ultrasound radiation into the inside of the living being and by measuring ultrasound radiation reflected by the inside of the living being, - an ultrasound sensor for being arranged at the location of the brachytherapy catheter (12; 112; 212) within the living being, wherein the ultrasound sensor is adapted to generate an ultrasound signal based on ultrasound radiation sent by the ultrasound imaging device (40, 42) and received by the ultrasound sensor, - a position determining unit for determining the position of the ultrasound sensor based on the generated ultrasound signal and for determining the pose and shape of the brachytherapy catheter (12; 112; 212) within the living being and/or the position of the radiation source, when the introduction element (13; 113; 213) has been introduced into the brachytherapy catheter (12; 112; 212), based on the determined position of the ultrasound sensor. - an ultrasound imaging device (40, 42) for generating an ultrasound image of the inside of the living being by sending ultrasound radiation into the inside of the living being and by measuring ultrasound radiation reflected by the inside of the living being, - an ultrasound sensor for being arranged at the location of the brachytherapy catheter (12; 112; 212) within the living being, wherein the ultrasound sensor is adapted to generate an ultrasound signal based on ultrasound radiation sent by the ultrasound imaging device (40, 42) and received by the ultrasound sensor, - a position determining unit for determining the position of the ultrasound sensor based on the generated ultrasound signal and for determining the pose and shape of the brachytherapy catheter (12; 112; 212) within the living being and/or the position of the radiation source, when the introduction element (13; 113; 213) has been introduced into the brachytherapy catheter (12; 112; 212), based on the determined position of the ultrasound sensor.