The present invention seeks to provide a radiotherapeutic apparatus that mitigates the various problems found in the techniques such as tomotherapy, IMAT, IMRT and the like. It provides a radiotherapeutic apparatus comprising a source of radiation whose output is collimated by a multi-leaf collimator, and a patient support, the source being rotateable around the support and the support being translatable along the axis of rotation, thereby to move the source helically relative to a patient on the support. The leaves of the MLC are preferably oriented orthogonal to the axis of rotation, to simplify computation of the dose distribution. The apparatus thus moves the patient on the patient support system along the axis of rotation, in the longitudinal direction. Thus, the device has an effectively unlimited treatable volume in the longitudinal direction and avoids the limitations of IMAT and IMRT techniques whilst enabling the use of thin MLC leaves to give a high longitudinal resolution. The apparatus is preferably combined with an optimisation system providing a computational service similar to that provided for IMAT and IMRT devices. Essentially the same computational techniques could be used, with appropriate changes to the input conditions and characteristic equations. The long aperture length (compared to tomotherapy) makes the radiation delivery efficient and therefore the delivery of high doses a practicality; hypofractionation and radiosurgery therefore become possible over large treatable volumes.