Answer to Question #3040 Submitted to "Ask the Experts"

What is the effective half-life of Tc-99m DMSA and MIBI when used for a kidney and heart SPECT in nuclear medicine, respectively?

The models for these two compounds, as given in ICRP Publications 53 (ICRP 1988) and 80 (ICRP 1999) are fairly complicated as internal dose models go.

For Tc-99m DMSA (2,3-dimercaptosuccinic acid), ICRP 53 suggests that activity is cleared from the body with three components:

• 25% with a biological half-time of 2 hours, and an effective half-time of 1.5 hours;
• 25% with a biological half-time of 1.8 days, and an effective half-time of 5.3 hours; and
• 50% with an infinite biological half-time, and an effective half-time of 6 hours.

The overall clearance of the material appears thus to have about a five-hour effective half-time.

For Tc-99m MIBI (methoxyisobutylisonitrile), ICRP 80 gives distributions and half-times for many tissues. Many have a fast clearing component with a 2-7-hour biological half-time, which results in a 1.5-3.2-hour effective half-time. Other have a slow clearing component with about a 24-hour biological half-time, which results in a 4.8-hour effective half-time. Most of the material is associated with the faster component, so an overall effective half-time for clearance from the body might be around three to four hours.

Michael G. Stabin, PhD, CHP

References

• International Commission on Radiation Protection Publication 53: Radiation dose to patients from radiopharmaceuticals. Annals of the ICRP; 18:1-4; 1988.
• International Commission on Radiation Protection Publication 80: Radiation dose to patients from radiopharmaceuticals. Annals of the ICRP; 38:3; 1999.