A radiotherapy apparatus comprises a means for producing a beam of radiation directed along a beam axis and having a width in first and second directions transverse to the beam axis, a multi-leaf collimator for selectively limiting the width of the beam in at least the first direction, a block collimator for selectively limiting the width of the beam in at least the second direction, the block collimator comprising a diaphragm moveable into and out of the beam and having a thickness in the direction of the beam axis that varies. The diaphragm can have a front edge of greater thickness than at least one region behind the front edge. It can also have a spine region extending from a rear part thereof towards the front edge that is greater thickness than at least one region displaced laterally with respect thereto. Together, these can cover the areas that will not be fully shadowed by a dynamically moving MLC. A control means for the multi-leaf collimator can be arranged to extend leaves of the multi-leaf collimator to shadow regions of the beam that are blocked by a relatively thinner section of the diaphragm. This is made easier if the spine region extends from the rearmost part of the diaphragm, the spine region extends to the front edge of the diaphragm, the spine region is straight, the spine region is a central region of the diaphragm, and if the width of the spine region increases towards the front edge of the diaphragm. The present invention also relates to a radiotherapy apparatus comprising a multi-leaf collimator and a block collimator, the block collimator comprising a diaphragm with variable thickness, and to a block collimator for use in radiotherapy apparatus comprising a diaphragm moveable into and out of a beam, and having a thickness in the direction of the beam axis that varies.