Impact factors for the outcome of the first 131 I radiotherapy in patients with papillary thyroid carcinoma after total thyroidectomy.

OBJECTIVE: To investigate the impact factors of the outcome of the first 131 I treatment in patients with papillary thyroid carcinoma (PTC) after total thyroidectomy.

METHODS: Three hundred and fifty-three patients [256 females, 97 males, average age (43.58 ± 12.33 years)] with PTC after total thyroidectomy who underwent 131 I treatment from July 2014 to August 2017 were retrospectively analyzed. Curative efficacy of 131 I treatment was assessed 6 months afterward. Therapeutic outcome was determined according to thyrotropin (TSH)-stimulated thyroglobulin (sTg) level, 131 I diagnostic whole-body scan (Dx-WBS) after 131 I treatment and other imaging modalities. Twelve possible factors affecting the therapeutic outcome of 131 I treatment including patients' gender, age, interval between surgery and 131 I treatment, primary tumor size and extrathyroidal extension (ETE), number and range of primary tumor lesions, result of 99m TcO4 - thyroid scan, number of metastatic lymph nodes (LN), pre-treatment laboratory measurements [TSH, sTg and Tg antibody (TgAb)], therapeutic dose of 131 I and result of 131 I post-treatment whole-body scan (Rx-WBS) were analyzed using univariate and multivariate logistic regression. The receiver operator characteristic (ROC) curve and diagnostic cutoff value were analyzed to evaluate the predictive value of the significant quantitative impact factors for the outcome of 131 I treatment.

RESULTS: The curative rate of the first 131 I treatment in patients with PTC after total thyroidectomy was 62.32% (220/353). Univariate analysis indicated that gender, age, number and range of primary tumor lesions, number of metastatic LN, pre-treatment sTg and TgAb, therapeutic dose of 131 I and result of 131 I Rx-WBS (all P < 0.05) were significant factors affecting the outcome of 131 I treatment. Multivariate analysis revealed that the numbers of metastatic LN (regression coefficient = 1.170) and sTg (regression coefficient = 0.280) were significant impact factors (all P < 0.001). The regression equation was: Logit P = - 3.997 + 1.170 × number of metastatic LN + 0.280 × sTg (χ2  = 210.68, P < 0.001). Taking sTg as a predictive factor for the outcome of the first 131 I treatment, the area under the curve (AUC) of ROC for sTg was 0.917 (95% CI 0.890-0.944). The cutoff value of sTg was 2.69 ng/mL with a sensitivity of 92.48% (123/133) and specificity of 74.09% (163/220).

CONCLUSION: Patients with PTC after total thyroidectomy with low pre-treatment sTg level and few lymph node metastases are more likely to be cured by the first 131 I treatment.