A radiotherapy apparatus is described which includes a source of radiation for generating a therapeutic beam towards a target region of a patient. A collimation apparatus is configured to act on the beam in a plane transverse to the beam axis. An imaging apparatus is configured to obtain imaging data of the target region. A control apparatus is configured to receive the imaging data from the imaging apparatus and control the collimation apparatus in dependence thereon. The imaging data includes at least one two-dimensional slice image including only a single two-dimensional slice image oriented in any one direction and which includes the target region.
