QUALITY ASSURANCE OF RADIATION DOSAGE - USEFULNESS OF REDUNDANCY

Regulations for maintaining the calibration of radiation-measuring instruments generally require that they be recalibrated periodically, and some also specify periodic spot-checks. There is a finite probability that changes in the instrument sensitivity will occur between these calibrations or periodic checks. Routine spotchecks of the output of a therapy unit, made with a given instrument, should detect changes in either device. Comparisons of reliability obtained by these and other procedures for checking on instrument sensitivity are made using calculations based on estimates of the mean time between failures for sources and instruments. Evidently, significantly greater reliability and efficiency should result from considering each machine spot-check nd recalibration as a check on the stability of both the machine and the instrument, as opposed to placing complete dependence upon the instrument during the period between separate checks of its sensitivity.