ABSTRACT
Resistance of metastatic lymph nodes [LNs] to high dose I-131 therapy is associated with high morbidity in patients with differentiated thyroid cancer. We evaluated the role of F-18 FDG PET/CT in the prediction of resistance to high dose I-131 therapy in patients with papillary thyroid cancer. The subjects were 307 patients who underwent total or near total thyroidectomy followed by high dose [5.55-6.66 GBq] I-131 therapy. We divided the patients into three subgroups by visual assessment of regional LNs: FDG-avid LNs with a malignant shape on CT [PET/CT-positive group], FDG-avid LNs with a benign shape on CT [PET/CT-intermediate group] and no FDG-avid lesion [PET/CT-negative group]. We measured the maximum SUV [SUVmax] of FDG-avid LNs in each patient. The presence or absence of focal increased uptake of I-131 was evaluated by whole body scan [WBS], and was denoted as WBS-positive group or WBS-negative group, respectively. Resistance to therapy was defined as presence of thyroglobulin [Tg] in serum [Tg >/= 1.0 ng/ml] 3-6 months after I-131 therapy. Univariate and multivariate analyses were performed to determine the relationship between resistance to I-131 therapy and various clinico-pathologic variables. PET/CT-positive, intermediate, and negative groups included 20 [6.5%], 44 [14.3%] and 243 [79.2%] patients, respectively. The mean SUVmax was significantly higher in the PET/CT-positive group than that of the PET/CT-intermediate group [4.6 vs.2.7, P <0.001]. Univariate analysis revealed that the PET/CT-positive group [P <0.001], T2-4 stage [P <0.001], N1b stage [P = 0.001], lower dose [5.55 GBq] of I-131 [P <0.001], and the WBS-positive group [P = 0.029] were associated with resistance to therapy. In multivariate analysis, the PET/CT-positive group, lower dose of I-131, N1b stage, and T2-4 stage remained significant with odds ratios of 10.07 [P <0.001], 3.82 [P <0.001], 3.58 [P = 0.001], and 2.53 [P = 0.009], respectively. FDG-avidity and malignant shape of cervical LNs on pre-therapy FDG PET/CT were a strong risk factors predicting resistance to high dose I-131 therapy. A lower dose of administered I-131 [5.55 GBq] and more extensive tumors [T2-4 and N1b] were also associated with resistance to high dose I-131 therapy