The present disclosure relates to a method for controlling a magnetic resonance imaging guided radiation therapy apparatus comprising a magnetic resonance imaging system. The method comprises: acquiring magnetic resonance data using the magnetic resonance imaging system and the pulse sequence from an imaging volume segmenting the magnetic resonance data into a plurality of segments indicating respective tissues in the imaging volume creating a bulk electron density map of the imaging volume from the plurality of segments displaying the bulk electron density map and radiation dose distributions for the plurality of segments on a graphical user interface, wherein the radiation dose distributions are determined using the bulk electron density map receiving a modification signal for modifying at least a first segment of the segments recreating the bulk electron density map using the modified first segment, and recalculating the radiation dose distribution using the bulk electron density map redisplaying the bulk electron density map and the radiation dose distributions on the graphical user interface.
