The stability of teletherapy-beam symmetry with gantry angle

The authors describe a rapid means of evaluating the stability of beam symmetry as the gantry angle is varied. Two radiation detectors, a ratio circuit, and an X-Y recorder are used. The teletherapy machine is operated in the rotational mode, and the ionization-current ratio is recorded as a function of the gantry angle. This test can provide an early warning of beam-symmetry instabilities.