Question

Why can the dose measured in the dose calibrator be different from amount listed on the patient’s syringe from the radiopharmacy? (Think about time, decay, and acceptable ranges)

Answer 1

Variations in source geometry

• When the activity is drawn into a syringe, the source geometry will be different from that in a vial.
• The composition of the container, thickness, and distribution will affect the measurement.
• The self-absorption of the emitted radiation will change as the source volume changes.

Source position

• The manufacturer’s source holder is designed to keep the source in the area of maximum response on the vertical axis of the well.
• Variations in response due to changes in vertical height or horizontal position of a few millimeters are usually insignificant

Source adsorption

• Certain radiopharmaceuticals have been observed to adsorb to the surface of the container.
• The adsorbed activity can be a significant percentage of the total.
• The possibility of activity adsorption should be considered whenever the facility uses syringes from a different manufacturer.

The major advantage of this technique is that it directly tests the accuracy of the dose calibrator for measuring clinical radionuclides without requiring individual and expensive calibrated sources for the measurements. Although we did not observe radionuclide dependence of a calibration source, we prefer to use 133Ba since it emits low and high energy photons has a long half-life and detects all changes in dose calibrator chamber absorption characteristics over long periods of time. The procedure takes minimal time and no source investment is required since any source currently used for dose calibrator accuracy testing can be employed.