Metastatic differentiated thyroid cancer with negative serum stimulated Tg but positive post-therapeutic 131 I-SPECT/CT scintigraphy: a single-center retrospective study

Purpose This study aimed to describe the characteristics of patients with metastatic differentiated thyroid carcinoma (DTC) who had positive 131I-scintigraphy but negative stimulated thyroglobulin (sTg), and to evaluate their short-term response to radioiodine therapy (RAI). Methods A total of 2250 consecutive postoperative DTC patients, who underwent RAI treatment from July 2019 to June 2022, were analyzed retrospectively. The target group was defined as stimulated Tg < 2 ng/mL with TgAb < 100 IU/mL but with post-therapeutic 131 I-SPECT/CT metastases. The characteristics of these patients were analyzed and the metastatic profiles were compared with TgAb positive or sTg positive ones. A cross-sectional efficacy was evaluated 6–12 months after the RAI therapy and the treatment course until the end of the study was recorded. Results 105 (4.67%) DTC patients were post-therapeutic 131 I-SPECT/CT positive and sTg negative (target group). Metastatic profiles were found significant differences between sTg negative and sTg positive ones ( P < 0.001). Excellent response (ER) was achieved in 72.4% of the target group between 6–12 months of cross-sectional efficacy assessment, compared with only 12.8% in sTg positive ones ( P < 0.001). The majority of the target group didn’t require aggressive treatment in short-term follow-up compared with sTg positive group ( P < 0.001). Conclusion The percentage of DTCs with negative sTg but positive post-therapeutic 131 I-SPECT/CT was relatively low, but still significant. Moreover, the majority of these patients showed an ER to RAI and may not require the next course of therapy. Long-term follow-up is still necessary to assess recurrence and adapt surveillance in these patients.