A radiotherapy apparatus and method, comprising a source of radiation for delivering pulses of radiation at a base frequency; and a control unit configured to generate a time-averaged power level of x by generating an nth pulse when n is equal to the rounded value of an integer multiple of 1/x, where x is a value between 0 and 1 representing a proportion of a maximum power output of the radiotherapy apparatus. The pulses may form a repeating pulse train pattern, and the control unit may comprise a processing unit configured to provide a pulse train pattern having a number of available slots, the slots being marked as a permitted pulse or a suppressed pulse.