Endocrine Abstracts

Objectives: Evaluate the benefit of postsurgical radioactive iodine (RAI) in patients with differentiated thyroid cancer (DTC) at low to intermediate risk of recurrent disease in whom not routinary use of RAI is recommended

Methods: We retrospectively evaluated 1316 patients with DTC diagnosed between 2009-2015. ATA low (LR) and intermediate (IR) risk were included in the study (PTC: tumor size > 1 cm, microscopic extrathyroidal invasion, <5 microscopic N1 (<2 mm), >5 N1 of <3 cm, aggressive histology, vascular invasion; FTC minimally invasive with tumor size > 1 cm). These patients were categorized into either treated or untreated with I-131. The response to therapy was evaluated by ongoing ATA risk stratification system. Treatment effect was assessed by inverse-probability weighted regression adjustment (IPWRA), by balancing the distribution of factors influencing treatment assignment between I131-treated or untreated patients.

Results: A total of 469 patients (119 males and 350 females) were selected. The mean age at diagnosis was 46.1±14.3 years old. 328 patients (69.9%) were treated with I131 while 141 (30.1%) untreated. In the overall group, biochemical or structural disease was observed in 44 (9.4%) patients after a median time of 17.5 months from the diagnosis. Persistent/recurrent disease was more frequent in I131-treated than in untreated patients (12.5% vs 2.1%, respectively, P < 0.001). This result was in line with the bias that I131-treated patients displayed a more aggressive pathology at diagnosis. By the inverse-probability weighted regression adjustment (IPWRA) analysis, the estimated percentage of recurrent disease was 10% (95% CI= 6.3-12.9%) in I131-treated and 16% (95% CI = 11.1-20.71%) in untreated patients (P = 0.02). Hence, if all patients would be treated with I131, the estimated risk of relapse would be reduced by 40% (RR= 0.6; 95% CI = 0.40-0.92, P = 0.018). Multivariate logistic regression analysis identified as factors independently associated with persistent/recurrent disease: pN1 (OR=3.56; 95% CI 1.55-8.17), male sex (OR=2.45; 95% CI 1.12-5.36) and microscopic extrathyroidal extension (OR=3.28; 95% CI 1.5-7.15).

Conclusions: Radioiodine administration in LR and IR is frequently defined on a case-by-case basis as suggested by retrospective studies. To make the outcome conditionally independent of the treatment assignment, in our matching method we observed that treatment with I131 in low/intermediate DTC reduces the absolute risk of persistent/recurrent disease by 6%. Only prospective clinical trial may definitively answer the question whether consider or not consider I131 therapy for low-to-intermediate DTC. Until then, a careful evaluation of the whole range of “aggressive” features of the tumor should guide RAI decision making.