The ability to provide a fine resolution variation in the dose rate of radiotherapeutic apparatus would allow delivery times of VMAT and related techniques to be reduced significantly. A radiotherapeutic apparatus is described, comprising a source of radiation able to issue pulses of radiation at a base frequency, and a control apparatus arranged to permit pulses provided these match a pulse pattern chosen to achieve a selected time-averaged power level, the chosen pulse pattern being one selected from a plurality of pulse patterns that include patterns consisting of trains of pulses that include, relative to the base frequency, at least every nth and every mth pulse where n≠m and the nth and mth pulses are non-coincident. Essentially, such an apparatus issues a pulse if it is the nth or the mth or the kth (etc) where n, m, k . . . are selectable independently. The pulse pattern can be obtained resolving the selected power level as an integer representable as a binary number, and then inserting an nth pulse if the binary number has a 1 digit at the qth power of 2 where q is the nth digit of the binary carry sequence. The binary carry sequence can be repeated modulo the number of possible output power levels, assuming that the selected power level is one selected from a range of possible output power levels. The control apparatus can a memory in which a table is retained, the table containing the sequence of pulses to be issued. The table can be freshly populated with a sequence when a power level is selected, or the table can contain sequences for a plurality of (preferably all the) power levels and the control unit can be adapted to select a desired sequence.
