Recent advances in treatment planning and in the apparatus able to deliver such plans has called for the dose rate, i.e. the instantaneous power output of the radiation source, to be varied with time. This presents a difficulty in that the checking systems must monitor a varying power level against a varying valid range. We therefore monitor, instead, the energy of the individual pulses that form the beam. Known checking systems average out many pulses to determine the recent average power output by checking an ionization chamber every 100 ms or so. By reducing that time to less than a few milliseconds, a single pulse can be captured. The usual manner of varying the output of a radiation source of this type is to vary the pulse repetition frequency (PRF) and therefore the measured power output will remain constant notwithstanding changes to the time-averaged power output, and can be compared to a standard.