Systems and methods for Image-Guided Radiation Therapy (IGRT), particularly but not exclusively when practised with a magnetic resonance imaging (MRI) system, which utilises an iterative approach which adjusts a treatment plan based on inter- or intra-fraction images to significantly improve the accuracy of the radiation delivered during the overall treatment. The prescribed dose of radiotherapeutic radiation is mapped onto the patient's anatomy using an image acquired of the region which is to be the target for radiotherapeutic radiation. Following beam-angle-optimisation, fluence optimisation and segmentation, the efficiency of delivery of each segment is determined using an objective function, and the segments ranked according to their efficiency. The plan proceeds with the choice of the most efficient segment (or segments) to be delivered first. When this radiation has been delivered, having been tracked to establish its distribution, this delivered distribution can be subtracted from the original prescribed dose and the process repeated so that the delivered radiation gradually converges on the original prescribed dose.