The invention provides for a medical instrument (400, 500, 600, 700) comprising a magnetic resonance imaging system (404) and an external beam radiotherapy system (402) for irradiating a target zone (438) of a subject with a beam (442) of ionizing radiation within the imaging zone. The medical instrument further comprises a processor (448) for controlling the medical instrument. Execution of instructions cause the processor to: acquire (100, 200) first magnetic resonance data (458)； reconstruct (102, 202) a first magnetic resonance image (460) from the first magnetic resonance data； receive (104, 204) planning data (462), 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； and calculate (108, 208) an external beam dosage plan (468) and a brachytherapy dosage plan (468) using the spatially dependent radiation dose and the first magnetic resonance image.
