Radiation Exposure During Remnant Ablation

Iodine biokinetics and radioiodine exposure after recombinant human thyrotropin-assisted remnant ablation in comparison with thyroid hormone withdrawal.

Taieb D , Sebag F , Farman-Ara B , Portal T , Baumstarck-Barrau K , Fortanier C , Bourrelly M , Mancini J , De Micco C , Auquier P , Conte-Devolx B , Henry JF , Mundler O

J Clin Endocrinol Metab.2010 Jul;95(7):3283-90. Epub 2010 Apr 14.

Context

A few prospective studies have evaluated the use of recombinant human TSH (rhTSH) for radioiodine remnant ablation.

Objective

Our objective was to compare the effects of the both TSH regimens on iodine biokinetics in the thyroid remnant, dosimetry, and radiation protection.

Design

We conducted a prospective randomized study.

Materials and Methods

Eighty-eight patients were enrolled for radioiodine ablation to either the hypothyroid or rhTSH arms. A whole-body scan was performed at 48 and 144 h after therapy. Dose rates were assessed at 24, 48, and 144 h. Urinary samples were obtained during the first 48 h. Thyroglobulin was assessed before and after therapy. Iodine biokinetics in the remnants were calculated from gamma-count rates. Radiation-absorbed dose was calculated using OLINDA software. Exposure estimation was based on a validated model.

Results

The effective half-life in the remnant thyroid tissue was significantly longer after rhTSH than during hypothyroidism (P = 0.01), whereas 48-h (131)I uptakes and residence times were similar. After therapy, thyroglobulin release (a marker of cell damage) was lower in the rhTSH arm. The mean total-body effective half-life and residence time were shorter in patients treated after rhTSH. Residence time was also lower for the colon and stomach. Absorbed dose estimates were lower in the rhTSH arm for the lower large intestine, breasts, ovaries, and the bone marrow. Dose rates at the time of discharge were lower in the rhTSH group with a reduction in cumulative radiation exposure to contact persons.

Conclusions

In comparison with thyroid hormone withdrawal, rhTSH is associated with longer remnant half-life of radioactive iodine while also reducing radiation exposure to the rest of the body and also to the general public who come in contact with such patients

Comment

this is a very careful study, and conclusions are well based on observations. However it may be useful to reconsider several aspects of the study. The hypothyroid patients were without medication for an unstated interval, which was much longer than needed to elevate their TSH above the generally accepted level of 30miU/ml. Final TSH levels averaged 110mU/L Uptakes were significantly higher in the withdrawal group, and thyroid half life shorter, probably because of the intense and prolonged TSH stimulation. In relation to this, post-therapy serum TG levels (taken as a measure of thyroid damage,) were higher in the withdrawal group. In both groups observed whole body radiation levels were very low (all < 0.1 rad) and average total exposure to the public was very low (all under 0.02rads/treatment).

  • Concerns with this study are that the treatment plan did not compare the use of any of the published methods for inducing mild hypothyroidism, and employed higher radiation doses than generally needed. Patients can be made sufficiently hypothyroid for therapy by using any one of several published methods to produce partial hypothyroidism, with resultant TSH levels in the 30-50mIu/ml range, and avoiding damage to “quality of life”. As shown in numerous reports, ablation can be as effectively accomplished with 131-I doses of 30-50 mCi, which would automatically reduce personal and public radiation exposure by half or more. We await, and need, a similar careful study comparing patients prepared with rhTSH to preparation with mild hypothyroidism, and reduced 131-I dosage. These changes could benefit many individuals receiving 131-I remnant ablation.Leslie J De Groot, MD