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Internal Dose Calculation for ^99mTc

New York

ABSTRACT

The total local energy deposited per distintegration, _ß for ^99mTc is 14 kev per disintegration. The specific gamma-ray constant, , for ^99mTc is 0.72 R-cm²/mc-hr; however, 29 per cent of this value is due to K and K_ß x-rays which make up only 1.2 per cent of the total photon energy emitted by ^99mTc. Standard methods of calculating the gamma component of the absorbed dose for ^99mTc using the geometrical factor and yield results which underestimate the absorbed dose by 16 to 30 per cent compared to calculations based on Monte Carlo techniques which take into account the energy dependence of µ_air and the inflated value of . The Monte Carlo technique was used in making the absorbed dose estimates in this paper.

Absorbed dose estimates were made for ^99mTc as (a) TcO₄^– for brain scanning, as compared to ¹³¹I labeled serum albumin and ²⁰³Hg and ⁹⁷Hg labeled Neohydrin; (b) ^99mTc labeled serum albumin for placental scanning, as compared to ¹³¹I labeled serum albumin; and (c) the ^99mTc sulfur colloid for liver and bone marrow scanning as compared to colloidal ¹⁹⁸Au, colloidal ¹⁹⁹Au and ¹³¹I aggregated albumin. Some of the biological problems encountered in calculating the absorbed dose are discussed.

FOOTNOTES

¹ U.S. Department of Health, Education, and Welfare Public Health Service, National Institutes of Health Clinical Center, Department of Radiation Safety, Bethesda, Maryland, 20014.

² Manuscript written during period of service at the National Institute of Health. Present address: Hospital for Special Surgery, Cornell Medical Center, 535 East 70th Street, New York 21, New York.