Second radioiodine treatment hardly benefits TT-DTC patients with radioiodine-negative metastases on initial post-therapeutic whole-body scans.

Background: The effect of second ¹³¹I treatment (RT) in totally thyroidectomized differentiated thyroid cancer (TT-DTC) patients with true-positive thyroid beds and false-negative metastasis (TB+/M-) on initial post-therapeutic whole-body scan (Rx-WBS) remains unknown.

Methods: TT-DTC patients with TB+/M- on initial Rx-WBS receiving and not receiving second RT were categorized into group A and group B, respectively, while patients with ¹³¹I-avid metastasis receiving second RT were referred to as group C. Biochemical remission (BR) was defined as a decrease of ≥25.0% in thyrotropin-suppressed thyroglobulin (Tgon) level, while the structural response (SR) was determined by the change in the size of the largest lesion.

Results: In total, 145 patients were eligible. In group A, the median Tgon measured 3.3 ng/mL before and 3.0 ng/mL at 4 months after second RT (P=0.307), yielding a decrease in the median Tgon (∆Tgon%) of 13.3%, a BR rate of 36%, and an insignificant SR, which were comparable to those in group B. In group C, however, a median ∆Tgon% of 37.8% and a BR rate of 64% were obtained, which were significantly higher than those in group A (P=0.038 and 0.022, respectively), with SR distributions similar to those in group A. In addition, ¹³¹I uptake in the neck was not statistically associated with the detection of metastasis on initial Rx-WBS.

Conclusions: This controlled study demonstrated a subtle response to second RT in TT-DTC patients with TB+/M- on initial Rx-WBS, representing a meaningful advancement in avoiding ineffective repeated RT.