Percent 48-hr whole body retention for estimating maximum tolerated activity of 131-I for the treatment of well-differentiated thyroid cancer: The effect of rhTSH stimulation vs thyroid hormone withdrawal on a bi-exponential dosimetry model

Abstract

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Objectives: Previously (JNuclMed 2006:47:324P), we reported a bi-exponential function (BF) that could estimate the maximum tolerated activity (MTA) of 131-I for the treatment (tx) of well-differentiated thyroid carcinoma (WDTC). This BF was (1) easier to use than whole body dosimetry (WBD), (2) based on a single measurement of the percent (%) whole body retention (WBR) of 131-I obtained 48 hrs post dosing , and (3) performed after thyroid hormone withdrawal (THW). Since the biokinetics of iodine may be different using rhTSH injections than THW, the objective of this study was to evaluate if the same BF would still apply for patients in whom rhTSH was used. Methods: A review was conducted of our patients (>16 y.o.) who (1) were suspected of recurrent WDTC, and (2) had rhTSH-stimulated WBD between 2/00 and 12/06. The MTA was determined using a modified Benua-Leeper WBD approach. From the WB clearance curve, the %WBR at 48 hrs was interpolated from the closest time points. For comparison, the rhTSH-BF and THW-BF were both calculated assuming MTA/BSA=a*exp(b*R)+c*exp(d*R) where R is the %WBR at 48hrs. The mean and maximum relative differences between the rhTSH-BF and THW-BF were determined over the range of %WBR from 3-25%. Results: Twenty-six patients (19F,7M) were studied. Normalizing the MTA to the body surface area (BSA) essentially removes the gender dependence. The coefficients for both the THW-BF and (rhTSH-BF) are shown in the table. [table] The mean %WBR for rhTSH was 8.8% (range 1.02-40.8%), which was different (P<0.001) from 15.3% (range 2.6-49.8%) for the THW group. Likewise, the mean normalized MTA was also different (P<0.001); 310(rhTSH) vs 226(THW) mCi/m². However, the two BF are very similar (P<0.01) with a mean % deviation of 3.5% (range -6.6% to +7.6%). Conclusions: Although the biokinetics are different the rhTSH-BF and THW-BF are essentially identical, and the later may be used to help estimate the dosage of 131-I for the tx of WDTC using either THW or rhTSH.