The stability of the X-ray beam from an 8 MV linear accelerator designed for radiotherapy

A study has been made of the stability of the X-ray beam from an 8 MV medical linear accelerator (M.E.L. Equipment Co. Ltd., type SL75 Super). The flatness of the beam and the intensity of the beam as measured by the built-in monitor are found to suffer small changes with gantry angulation and with time. Measurements have been extended over a year during which the accelerator has been in daily use for radiotherapy. The causes of the changes in the beam and the function of the stabilising circuits which largely counteract such changes are outlined. The resulting complex system must be controlled by radiation measurements on the beam. Simple measuring jigs have been made for the purpose and are described; they allow thimble ionisation chambers to be accurately positioned in the beam during gantry angulation and for daily, weekly and monthly checks. By routine use of these devices the changes of the beam have been monitored and appropriate control adjustments made to the accelerator when necessary. The accuracy of the accelerator for radiotherapy purposes, when the techniques described are used for routine control, is examined and discussed.