An apparatus is disclosed, which comprises a radiotherapy delivery apparatus having a radiation source for emitting a beam of radiation and rotatable around an axis intersecting the beam through a range of at least x degrees, an imaging device, and a control unit for controlling the source and imaging device. The apparatus further comprises a treatment planning computer configured to receive a first image of a patient, and receive parameters defining the operations of the radiotherapy delivery apparatus. The parameters include an available range of rotation for the source defined as x-e, where e>0. The treatment planning computer is further configured to generate a treatment plan for the patient based on the received image and parameters. The control unit determines a patient position using the imaging device, determines a rotation r of the patient around the axis relative to the first image, and compensates for the rotation by offsetting the source by a rotation equal to r when r