Some factors affecting the calibration of radionuclide calibrators--I. 99mTc.

In the Winnipeg area a centralized radiopharmacy serves several Nuclear Medicine departments supplying all their 99mTc radiopharmaceuticals on a daily basis. It was observed that there was very poor consistency between the assays as determined by the Radiopharmacy and those determined in the various Nuclear Medicine departments. An attempt to "correct" the various calibrators using a 57Co "mock" 99mTc source resulted in an aggravation of the problem, and an investigation of relative responses of various calibrators to the "mock" relative to a 99mTc source was undertaken, together with an investigation of the influences of various environmental conditions on the calibrator function. In many Nuclear Medicine departments technologists sensitive to concerns of radiation exposure have stacked lead shielding around the calibrator as added protection. However if the calibrator is not calibrated with sufficient lead shielding, such additional shielding will increase the ionization within the chamber, resulting in an elevated assay being obtained due to the lead X-rays and back-scattered radiation. In the absence of lead shielding, similar effects, although smaller in magnitude, are observed from other environmental factors such as the proximity of a wall or a person. Thus in selecting a calibrator, one which comes from the manufacturer, with a calibration certified by the manufacturer, incorporating an adequate amount of lead shielding to prevent additional back-scatter re-entering the ionization chamber should be considered essential unless a complete on-site recalibration using accurately calibrated samples of the radionuclides in question is planned.