A medical instrument (400, 500, 600, 700) comprising: - a magnetic resonance imaging system for acquiring magnetic resonance data (458, 516) from a subject within an imaging zone; - an external beam radiotherapy system for irradiating a target zone with a beam of ionizing radiation within the imaging zone; - a memory for storing machine executable instruction; - a processor for controlling the medical instrument, wherein execution of the instructions cause the processor to: - acquire (100, 200) first magnetic resonance data from the subject using the magnetic resonance imaging system; - reconstruct (102, 202) a first magnetic resonance image from the first magnetic resonance data; - receive (104, 204) planning data, wherein the planning data specifies a spatially dependent radiation dose for the target zone; - register (106, 206) the planning data to the first magnetic resonance image; characterised in that execution of the instructions cause the processor to: - calculate (108, 208) an external beam dosage plan and a brachytherapy dosage plan using the spatially dependent radiation dose and the first magnetic resonance image and where - the external beam dosage plan is a plan or commands for specifically irradiating the target zone using the external beam radiotherapy system and - the brachytherapy dosage plan is a plan for irradiating the target zone using brachytherapy, the brachytherapy dosage plan including placement position of radioactive source(s) mounted on (a) brachytherapy-catheter(s) and time duration the radioactive source will remain at these position(s). - a magnetic resonance imaging system for acquiring magnetic resonance data (458, 516) from a subject within an imaging zone; - an external beam radiotherapy system for irradiating a target zone with a beam of ionizing radiation within the imaging zone; - a memory for storing machine executable instruction; - a processor for controlling the medical instrument, wherein execution of the instructions cause the processor to: - acquire (100, 200) first magnetic resonance data from the subject using the magnetic resonance imaging system; - reconstruct (102, 202) a first magnetic resonance image from the first magnetic resonance data; - receive (104, 204) planning data, wherein the planning data specifies a spatially dependent radiation dose for the target zone; - register (106, 206) the planning data to the first magnetic resonance image; characterised in that execution of the instructions cause the processor to: - calculate (108, 208) an external beam dosage plan and a brachytherapy dosage plan using the spatially dependent radiation dose and the first magnetic resonance image and where - acquire (100, 200) first magnetic resonance data from the subject using the magnetic resonance imaging system; - reconstruct (102, 202) a first magnetic resonance image from the first magnetic resonance data; - receive (104, 204) planning data, wherein the planning data specifies a spatially dependent radiation dose for the target zone; - register (106, 206) the planning data to the first magnetic resonance image; characterised in that execution of the instructions cause the processor to: - calculate (108, 208) an external beam dosage plan and a brachytherapy dosage plan using the spatially dependent radiation dose and the first magnetic resonance image and where - the external beam dosage plan is a plan or commands for specifically irradiating the target zone using the external beam radiotherapy system and - the brachytherapy dosage plan is a plan for irradiating the target zone using brachytherapy, the brachytherapy dosage plan including placement position of radioactive source(s) mounted on (a) brachytherapy-catheter(s) and time duration the radioactive source will remain at these position(s).