Dosimetry during adjuvant 131I therapy in patients with differentiated thyroid cancer-clinical implications

Abstract

The activity of radioiodine (131I) used in adjuvant therapy for thyroid cancer ranges between 30 mCi (1.1 GBq) and 150 mCi (5.5 GBq). Dosimetry based on Marinelli's formula, taking into consideration the absorbed dose in the postoperative tumour bed (D) should systematise the determination of 131I activity. Retrospective analysis of 57 patients with differentiated thyroid cancer (DTC) after thyreidectomy and adjuvant 131I therapy with the fixed activity of 3.7 GBq. In order to calculate D from Marinelli's formula, the authors took into account, among other things, repeated dosimetry measurements (after 6, 24, and 72 h) made during scintigraphy and after administration of the therapeutic activity or radioiodine. In 75% of the patients, the values of D were > 300 Gy (i.e. above the value recommended by current guidelines). In just 16% of the patients, the obtained values fell between 250 and 300 Gy, whereas in 9% of the patients, the value of D was < 250 Gy. The therapy was successful for all the patients (stimulated Tg < 1 ng/ml and 131I uptake < 0.1% in the thyroid bed in follow-up examination). Dosimetry during adjuvant 131I therapy makes it possible to diversify the therapeutic activities of 131I in order to obtain a uniform value of D.

Absorbed dose coefficients for adult thyroid cancer patients undergoing radioiodine therapy

“…For example, although the thyroidal uptake of I-131 at 48 h after administration was reported to be numerically different between the rhTSH and THW groups, no statistically significant differences were observed because of the large inter-individual uncertainties (0.51 ± 0.7 vs. 0.91 ± 1.05%; P = 0.1 in [24] and 0.70 ± 1.04 vs. 0.92 ± 0.94%; P = 0.065 in [25]). In addition, the mean thyroidal uptake observed after rhTSH administration varied greatly depending on the subject group (0.51%, 0.7%, and 0.91% in [24,25], and [37], respectively). Nevertheless, the predictions of the rhTSH and THW models for extrathyroidal iodine, directly related to TIACs and absorbed doses for normal tissues, are within an acceptable range; for example, the predicted half-times of extrathyroidal iodine (12 vs. 15 h for the rhTSH and THW models, respectively) lie within the ranges of mean half-times observed (9.4-14.8 h vs. 12.4-17.1 h in rhTSH and THW groups, respectively, over a 144 h observation period) [24,25,38,39].…”