Dose–Response Relationship in Differentiated Thyroid Cancer Patients Undergoing Radioiodine Treatment Assessed by Means of ¹²⁴I PET/CT

Abstract

The dose-response relationship in a fixed-activity approach generally applied in the treatment of differentiated thyroid cancer was assessed using 124 I PET/CT. Methods: Pretherapeutic 124 I PET/ CT images of 47 patients scheduled for radioiodine therapy were retrospectively analyzed. 124 I PET/CT images were acquired 24 and 96 h after oral administration of approximately 28 MBq of 124 I-sodium iodide. Lesions were identified as thyroid remnants or metastases (lymph node, lung, bone). After a neoteric segmentation technique allowing accurate volume estimation down to the 124 I PET spatial resolution of 0.15 mL was applied, lesions were divided into a known-volume group and a small-volume group. For the known-volume group, average lesion-absorbed dose (AD) values were calculated, whereas for the small-volume group a minimum lesion AD was estimated. Lesion response was determined on the basis of 124 I PET/CT and 131 I SPECT/CT follow-up images. A lesion not detectable on any of the follow-up images was considered a completely responding lesion. Differences in lesion AD estimations between completely and incompletely responding lesions were evaluated by Mann-Whitney U test. Moreover, receiver-operating-characteristic curves were used to test the performance of pretherapeutic 124 I PET/CT lesion AD for prediction of complete lesion response. Results: In the approach of fixed radioiodine activity (3.0 ± 1.0 GBq), 89% of thyroid remnants and 69% of metastases responded completely. Except for the small-volume groups, the lesion AD of completely responding lesions was significantly higher than that of incompletely responding lesions. Using receiver-operating-characteristic curve analysis, it was shown that for the known-volume group, pretherapeutic 124 I PET/CT lesion dosimetry can be used as a prognostic tool to predict lesion-based 131 I therapy response with an area under the curve of 0.76 for remnants and 0.97 for metastases. The corresponding lesion AD threshold value maximizing correct complete response prediction was 90 Gy for remnants and 40 Gy for metastases. Conclusion: In a fixed-activity approach, a statistically significant dose-response relationship for both thyroid remnants and metastases using pretherapeutic 124 I PET/CT lesion dosimetry was found. The findings may be useful in patient management. Radi oiodine therapy is the standard adjuvant therapy after total thyroidectomy in patients with differentiated thyroid cancer (DTC) (1,2). The purpose of radioiodine therapy is 2-fold: ablation of thyroid remnants and treatment of radioiodine-avid metastases.The generally accepted threshold for lesion-absorbed dose (AD) to achieve high therapy response is 85 Gy for metastases and 300 Gy for thyroid remnants (3,4). To predict therapy response, a reliable AD estimation requires, inter alia, accurate lesion volume determination, which is hampered by the often small lesion volumes with respect to the spatial resolution of 131 I scintigraphy and lesion segmentation difficulties encountered in ultrasonography, CT, ...

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