Has come the day to do away with thyroid remnant ablation targeting 300 gray (Gy)?

Abstract

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Objectives: Twenty-four years ago (Maxon HR et al. N Engl J Med 1983 [1]) established absorbed dose thresholds for successful radioiodine-131 (I-131) treatment of thyroid remnants (300 Gy, N=30, p<0.05) and lymph node metastases (80 Gy, N=67, p<0.01) in thyroid cancer patients. Since then, these thresholds have been repeatedly quoted in related dosimetric context. The objective of this work was to review published quantitative studies and to investigate whether the literature, including a subsequent data of original authors, support initial proposal. Methods: Four papers on in-vivo dosimetric quantitation of I-131 therapy for thyroid cancer (Maxon HR et al. J Nucl Med 1992 [2], O'Connell MEA et al. Radiother Oncol 1993 [3], Samuel AM et al. J Nucl Med 1994 [4] and Lassmann M et al. J Nucl Med 2004 [5]) were analysed, together with two presentations of abstracts (Behr TM et al. Eur J Nucl Med Mol Imaging 2002 [6] and Medvedec M et al. Eur J Nucl Med Mol Imaging 2004 [7]). The data were adopted from available studies exactly in the form in which they had been published or presented and were analyzed using nonparametric statistics. Results: The absorbed dose of 300 Gy was not statistically proven to be the threshold for successful treatment [2, 3, 4, 5, 7]. Proposed threshold absorbed dose-rate of 3 Gy/h for thyroid remnant ablation [4] was not proven as well. The lowest study-specific thresholds for thyroid remnant ablation were derived as follows: 40 Gy (N=11, p<0.01) [5], 50 Gy (N=39, p<0.01) [3], 150 Gy (N=37, p<0.01) [6], 1.6 Gy/h .AND. 100 Gy [7] (N=40, p<0.05). The latter data has been further supported by the mean absorbed doses of therapeutic I-131 delivered for successful (147±128 Gy) and unsuccessful (41±16 Gy) thyroid remnant ablation (N=23, p<0.05), as presented in abstract by Haq M et al. Eur J Nucl Med Mol Imaging 2006. Conclusions: Quantitative in-vivo data published so far does not support a widely-accepted dosimetric approach to thyroid remnant ablation based on the threshold absorbed dose of 300 Gy. In contrast, intrathyroidal doses of 40-150 Gy appear to be the range of evidence based cut-off values required for significantly higher success-rate of thyroid remnant ablation with I-131 in postsurical thyroid cancer patients.