An Instrumented Shield System for Calibration of Technetium-99m

Abstract

Objective: The elution of ^99mTc from commercial ⁹⁹Mo- ^99mTc generators and the subsequent calibration of both the bulk activity and ⁹⁹Mo impurity can lead to significant extremity dose to the fingers, hands and forearms. The dose arises primarily from the need to handle intense, unshielded sources when calibrations are performed with re-entry type ion chambers. The objective of this study was to develop a dose calibration system that incorporated a fully-instrumented shield for assay of gamma-emitting radioisotopes without the need for handling unshielded sources.

Methods: Prototype instrumented shield systems were tested in routine calibration of ^99mTc eluted from commercial ⁹⁹Mo-^99mTc generators to determine the quality and reproducibility of measurements. Pilot experiments were conducted to compare the extremity dose burdens from calibrations with the instrumented shield system and with standard re-entry type ionization chambers.

Results: Decay measurements with ^99mTc show that the instrumented shield system provides calibrations with less than 2% systematic nonlinearity for sources of 5 x 10²-1.5 x 10⁵ MBq (15–4000 mCi). Systematic errors from source geometry variation, including changes in eluate volumes from 5–21 ml, are less than 1%. Typical calibration procedures with a re-entry type ion chamber produce dose burdens to the fingers of (2.7–10.8) x 10^-4 mSv GBq^-1 (1–4 mRem Ci^-1) of ^99mTc handled.

Conclusion: When calibrations were performed with the instrumented shield system, the dose received could not be distinguished within the sensitivity limit of TLD finger rings used in the measurements.