Comparison of efficacy and prognostic impact of adjuvant 131 I therapy at 3.7 GBq and 5.55 GBq for in DTC patients with unexplained sTg elevation

Background The optimal dose of adjuvant radioiodine(¹³¹I) therapy for differentiated thyroid cancer (DTC) remains controversial. This study aimed to determine the efficacy and prognostic impact of two doses of adjuvant ¹³¹I therapy (3.7 GBq and 5.55 GBq) in DTC patients with unexplained TSH-stimulated Tg(sTg) elevation.Methods Data for eligible patients with DTC who received adjuvant ¹³¹I therapy at our institution between January 2015 and December 2016 were retrospectively reviewed. The results of dynamic risk assessment of persistent and recurrent disease (PRD) and recurrence-free survival (RFS) were compared between the 3.7 GBq and 5.55 GBq ¹³¹I groups using the chi-squared test, Fisher’s exact test, log-rank test, and a Cox proportional hazards model.Results In total, 224 patients with DTC were enrolled. Six months after adjuvant ¹³¹I therapy, 132 patients(58.9%) had an acceptable response and 92 (41.1%) had an unacceptable response. After a median follow-up duration of 6.7 years (range, 6.0–7.9), 12 patients (33.33%) had persistent disease and 24 (66.7%) had recurrent disease. One patient died during follow-up. The 5-year RFS rate after ¹³¹I treatment was 91.7%. At 6 months after treatment, there was no significant between-group difference in efficacy or the incidence of PRD or RFS (P > 0.05). Univariate analysis revealed significant associations of ¹³¹I whole-body scan combined with ¹³¹I-WBS/SPECT results after ¹³¹I treatment, and number of ¹³¹I treatments with the incidence of PRD ( P = 0). In multivariate analysis, the number of surgeries (hazard ratio [HR] 3.147, 95% confidence interval [CI] 1.360–7.282, P = 0.007), number of ¹³¹I treatments (HR 0.046, 95% CI 0.020–0.108, P = 0.001), and efficacy at 6 months after ¹³¹I treatment (HR 0.287, 95% CI 0.113–0.732, P = 0.009) were significantly associated with RFS.Conclusions The efficacy of adjuvant ¹³¹I therapy and the prognosis in DTC patients with unexplained sTg elevation was unaffected by whether the dose is 3.7 GBq or 5.55 GBq. Prospective, large-scale, long-term and RCTs clinical studies are needed to confirm these findings.