RT Journal Article SR Electronic T1 Dose–Response Relationship in Differentiated Thyroid Cancer Patients Undergoing Radioiodine Treatment Assessed by Means of 124I PET/CT JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1027 OP 1032 DO 10.2967/jnumed.115.168799 VO 57 IS 7 A1 Roel Wierts A1 Boudewijn Brans A1 Bas Havekes A1 Gerrit J. Kemerink A1 Servais G. Halders A1 Nicolaas N. Schaper A1 Walter H. Backes A1 Felix M. Mottaghy A1 Walter Jentzen YR 2016 UL http://jnm.snmjournals.org/content/57/7/1027.abstract AB The dose–response relationship in a fixed-activity approach generally applied in the treatment of differentiated thyroid cancer was assessed using 124I PET/CT. Methods: Pretherapeutic 124I PET/CT images of 47 patients scheduled for radioiodine therapy were retrospectively analyzed. 124I PET/CT images were acquired 24 and 96 h after oral administration of approximately 28 MBq of 124I-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 124I 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 124I PET/CT and 131I 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 124I 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 124I PET/CT lesion dosimetry can be used as a prognostic tool to predict lesion-based 131I 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 124I PET/CT lesion dosimetry was found. The findings may be useful in patient management.