A method for dealing with the effects of inertia in a radiotherapy apparatus is disclosed. The method may include catering 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. The instructions delivered to the geometry items may reflect their inertia behavior and can therefore be followed very closely. This may indicate that a departure from that plan will be correspondingly more likely to indicate an error by the geometry item. When a geometry item needs to accelerate or decelerate, less error may arise and thus the error-checking regime may not necessarily make allowances for such departures from the intended path, thereby tightening the error tolerances. Error-checking may be safely carried out locally for each component.