Effective half life of ¹³¹I in thyriod cancer patients

Abstract

The oral administration of radioactive ¹³¹I is a standard treatment for thyroid carcinoma. Sufficiently long physical half-life (8.04 days) and beta-emission (364 keV) of ¹³¹I coupled with its efficient localization by the thyroid tissue provides a basis for use in the treatment of thyroid cancer. Regulatory guidance recommends the use of biological elimination as well as physical decay in calculating the confinement time to keep the effective dose equivalent to members of the public less than 5 mSv. Limits on the doses are recommended on calculations based only on the physical decay of the 1317, However as the thyroid tissues of these patients have either been surgically removed or ablated by prior doses of ¹³¹I the body compartments that would retain the iodine are relatively small and the effective half life ( Tₑ ᶠ ᶠ) of 1317 which combines the physical and biological decay could be short. Even so the practice at the National Cancer Institute (NCI) Maharagama is to discharge the patients after 5 days. This study reports on an analysis of a patient database of 60 administrations of ¹³¹I for thyroid cancer. 50% of the patients gave an effective half life in the range 21 -25 h. Patients treated at the NCI with 100 — 200 mCi are released only after 5 days. 56 patients in this study group gave dose equivalent values less than 5 mSv at the end of the third day so that they could be released after 3 days. However the ideal situation would be where discharge criteria for radiation safety purposes should be calculated on the basis of individual measurement.