A radiotherapeutic apparatus for delivering a prescribed dose of radiotherapeutic radiation to a target region of patient tissue comprising a source for producing a directable beam of therapeutic radiation, wherein an image depicting patient anatomical data of at least the target region has been acquired and the prescribed dose has been mapped onto the image as the desired dose distribution to be achieved in the target region, the apparatus being adapted and configured to: a) determine at least one beam angle corresponding to a direction from which the radiotherapeutic radiation is to be delivered from the source to the target region, and for each beam angle create a mask defining a beam outline which substantially matches an outline of the target region as seen from each beam angle; b) discretize the or each beam within its beam outline into a plurality of pencil beams; c) for each beam angle carry out a fluence optimisation process using pencil beam data from step b) and patient anatomical and target region data from the image to generate a distribution profile for a sub-dose to be delivered at that beam angle, the separate sub-doses from all the angles in combination substantially equalling the prescribed dose; characterised in that the apparatus is further adapted and configured to: d) segment each sub-dose distribution profile into separate segments, each segment comprising a plurality of pencil beams of matching fluence; e) determine the efficiency of all of the segments by splitting the fluence of each beam into a plurality of discrete intensity levels and the cross-sectional area of the segment into discrete sections, so as to derive the efficiency as a function of the discrete intensity level and the area of a section; f) deliver radiation in accordance with at least one of the most efficient segments of therapeutic radiation; g) track the radiation delivered in step f) to calculate the amount and distribution of radiation delivered during the or each segment relative to patient anatomical data from the image, and subtract this from the mapped prescribed dose to create a new prescribed dose, and adjust the sub-doses and/or part sub-doses remaining to be delivered accordingly, and h) repeat the steps above in accordance with one of the following: 1) from step c) to step g), or 2) from step a) to step g), or 3) from step b) to step g). a) determine at least one beam angle corresponding to a direction from which the radiotherapeutic radiation is to be delivered from the source to the target region, and for each beam angle create a mask defining a beam outline which substantially matches an outline of the target region as seen from each beam angle; b) discretize the or each beam within its beam outline into a plurality of pencil beams; c) for each beam angle carry out a fluence optimisation process using pencil beam data from step b) and patient anatomical and target region data from the image to generate a distribution profile for a sub-dose to be delivered at that beam angle, the separate sub-doses from all the angles in combination substantially equalling the prescribed dose; d) segment each sub-dose distribution profile into separate segments, each segment comprising a plurality of pencil beams of matching fluence; e) determine the efficiency of all of the segments by splitting the fluence of each beam into a plurality of discrete intensity levels and the cross-sectional area of the segment into discrete sections, so as to derive the efficiency as a function of the discrete intensity level and the area of a section; f) deliver radiation in accordance with at least one of the most efficient segments of therapeutic radiation; g) track the radiation delivered in step f) to calculate the amount and distribution of radiation delivered during the or each segment relative to patient anatomical data from the image, and subtract this from the mapped prescribed dose to create a new prescribed dose, and adjust the sub-doses and/or part sub-doses remaining to be delivered accordingly, and h) repeat the steps above in accordance with one of the following: 1) from step c) to step g), or 2) from step a) to step g), or 3) from step b) to step g). 1) from step c) to step g), or 2) from step a) to step g), or 3) from step b) to step g).