Radiation-absorbed dose from 201Tl-thallous chloride

Revised radiation dosimetry estimates for 201Tl-thallous chloride have been developed using new data specifically acquired to address the issue of testicular uptake of this agent and through reevaluation of extant data for biodistribution in other organs.

Quantitative testicular scintigraphy data of sequestered testes (body-background shielded) were obtained from 28 patients (56 studies) injected with 201Tl-thallous chloride at peak exercise. Previously published data for 15 patients injected at maximal exercise were reanalyzed to obtain updated biodistribution parameters for designated organs. Radiation dose was calculated according to the MIRD schema. Radiation dose to testes as a function of age was determined. Comparisons are made between organ dose estimates derived in this study and those previously published. The dose contributions of possible contaminants (200Tl, 202Tl, 203Pb) have been included. Estimates are provided of the dose component from these contaminants if injected at the time of the maximum recommended 5-d shelf life (as opposed to at the designated calibration time).

The radiation dose per unit administered activity to adult testes calculated in this study of 0.21 mGy/MBq (0.77 rad/mCi) is approximately a factor of 2 less than the value of 0.45 mGy/MBq (1.7 rad/mCi) previously accepted. The revised dose estimates for other organs show less variation from published values. The effective dose determined in this work is approximately 0.16 mSv/MBq (0.60 rem/mCi). Under the assumption of similar biokinetics as for the adult, the testes dose for children increases significantly as age decreases with a value of 7.5 mGy/MBq (28 rad/mCi) for a newborn. Contributions from radiocontaminants that may be encountered in the preparation of 201Tl-thallous chloride are shown to range from a fraction of a percent up to approximately 20% of the total dose for some organs, with the higher values arising from the long half-life contaminant 202Tl after a 5-d shelf life.

It is recommended that the dose values determined in this study be used when estimating the radiation dose to the adult testes from intravenous administration of 201Tl-thallous chloride. The potential for increased radiation dose per administered activity to the testes at younger ages should be evaluated before performing procedures on children. The presence of radiocontaminants in the product should be considered when estimating radiation dose and may add a significant contribution to dose dependent on the specific radionuclide and concentration at the time of administration.