A testing jig for sealed beam monitors.

The dose received by a patient during radiotherapy on a linear accelerator is monitored by an ionisation chamber. The beam monitor in a Philips SL-75 accelerator is a dry-nitrogen-filled sealed box containing a pair of parallel-plate transmission ionisation chambers. If the monitor develops an air leak the charge measured by the chambers will fluctuate with ambient conditions resulting in variations in treatment dose per fraction. This note describes a self-contained testing system to differentiate between sealed and leaking beam monitors, but this technique necessitates the removal of the monitor from the accelerator.