Sung M. Kim MD, Abstract Co-Author: Nothing to Disclose

Kanchan Kulkarni, Abstract Co-Author: Nothing to Disclose

Charles Michael Intenzo MD, Presenter: Nothing to Disclose

Objective of I-131 treatment in the thyroid cancer is to maximize radiation dose effect to the tumor site and simultaneously to minimize the whole body radiation dose. Optimal TSH level is a prerequisite to achieve effective radioiodine uptake. High TSH level increases radioiodine uptake in the remnant or tumor sites, but prolonged hypothyroid condition also is related to decrease renal clearance of I-131, which inadvertently increases whole body radiation burden during the treatment. This optimal TSH value is unknown, but current clinical practice recommends it at least 30 mIU/ml. The authors assessed the relationship of TSH response and its biologic clearance of I-131 in the thyroid cancer patients being treated with I-131 to optimize the TSH level in order to achieve treatment objectives.

A body iodine clearance rate was calculated for 37 patients treated in hospital with 100 to 200 mCi of Iodine-131. Monoexponetial curve fitting was applied to the data of dose rate obtained at time of administration of I-131 and at the time of discharge from the hospital. These clearance data were correlated to the TSH value before the treatment.

TSH level ranges from 30 to 132 mIU/ml, yielding 5-fold variance between the individuals. The mean value of 66 (95% CI: 80-81). Biologic clearance rate of I-131 ranges from 1.2 hr to 15.3 hr, yielding 15-fold variance. Mean biologic clearance was 7.41 hour (95% CI: 6.3 to 8.5). Regression analysis showed a trend that high TSH value is correlated to delayed biological clearance of I-131. By using 95% CI values of biologic clearance of I-131, regression analysis calculated an optimal TSH value ranging from 54 to 64 uIU/ml without undue whole body radiation dose .

Wide range of serum TSH levels and biologic clearance rates of I-131 are noted for the patients being treated with I-131 in the clinical practice. Optimal selection of TSH value is one of important treatment objectives without increasing whole body radiation dose.

Kim, S, Kulkarni, K, Intenzo, C, Optimal TSH Value before I-131 Treatment; Relationship between TSH Response and Body-Clearance Rate of I-131 for Thyroid Cancer Patients.  Radiological Society of North America 2004 Scientific Assembly and Annual Meeting, November 28 - December 3, 2004 ,Chicago IL. http://archive.rsna.org/2004/4413725.html