Distinguishing Patients With Distant Metastatic Differentiated Thyroid Cancer Who Biochemically Benefit From Next Radioiodine Treatment

BACKGROUND: Repeated radioiodine ((131)I) treatment (RT) are commonly performed in patients with (131)I-avid distant metastatic differentiated thyroid cancer (DM-DTC), but more precise indications remain indeterminate. This prospective study was conducted to explore predictors for biochemical response (BR) to next RT. METHODS: Totally thyroidectomized patients with (131)I-avid DM-DTC demonstrated by initial post-therapeutic whole body scan (Rx-WBS) were consecutively recruited. Repeated RTs were performed at a fixed dose and a fixed interval, which was terminated once a decline in thyroid stimulating hormone-suppressed thyroglobulin (Tg(on)) could not be achieved or Rx-WBS was negative. BR was evaluated by change rate of Tg(on) level (ΔTg(on)%). RESULTS: After exclusion of 27 ineligible courses, a total of 166 neighboring course pairs from 77 patients were established and utilized. Univariate and multivariate analyses showed that the maximum target/background ratio (T/B(max)) on the whole body scan and ΔTg(on)% derived from the former RT were independently associated to the latter one. In predicting biochemical remission, the positive predictive value (PPV) and negative predictive value (NPV) of T/B(max) at the cut-off value of 8.1 were 79.1% and 84.0%, respectively; whereas the PPV and NPV of ΔTg(on)% at the cut-off value of 25.3% were 70.8% and 77.1%, respectively. Notably, the PPV of combined T/B(max) ≥ 8.1 and ΔTg(on)% ≥ 25.3% increased to 87.7%; while the NPV of T/B(max) ≥ 8.1 or ΔTg(on)% ≥ 25.3% reached as high as 97.7%. CONCLUSIONS: This study revealed that combined use of the latest RT-derived T/B(max) and ΔTg(on)% may efficiently identify biochemical responders/non-responders to next RT, warranting management optimization of patients with (131)I-avid DM-DTC.