We have found a superior method for dealing with the effects of inertia in a radiotherapy apparatus. We propose to cater for inertia in advance by incorporating inertia factors into an output from a delivery control system which adapts the treatment plan by incorporating the inertia factors, or by including the inertia factors as a constraint in the treatment planning process. Therefore, instead of instructing the machine to make movements which assume perfect inertia-less behaviour by the geometry item and then compensating for this afterwards, the instructions delivered to the geometry items will reflect their inertia behaviour and can therefore be followed very closely. This, in turn, means that a departure from that plan will be correspondingly more likely to indicate an error by the geometry item. It will no longer be routine for errors to arise when a geometry item needs to accelerate or decelerate and thus the error-checking regime need not make allowances for such departures from the intended path. That, in turn, means that the error tolerances can be correspondingly tighter. Those tighter tolerances mean that the error- checking can safely be carried out locally for each component.