Thyroglobulin Value Predict Iodine-123 Imaging Result in Differentiated Thyroid Cancer Patients

SIMPLE SUMMARY: This study was prompted to assess the diagnostic performance of (123)I-Dx-WBS-SPECT/CT in the identification of incomplete structural response in the early follow-up of DTC patients and derived optimized basal-Tg thresholds as a yardstick for scintigraphic imaging. In this light, we reviewed the records of 124 low or intermediate-risk DTC patients who underwent thyroid surgery followed by radioiodine therapy. The response to initial treatments was evaluated 6–12 months after radioiodine therapy. According to 2015 ATA criteria, 87, 19 and 18 patients were classified to have excellent response (ER), indeterminate/incomplete biochemical response (BIndR/BIR) or structural incomplete response (SIR), respectively. Among 37 patients with less than ER, 18 had a positive (123)I-Dx-WBS-SPECT/CT. Interestingly, metastatic disease noted at (123)I-Dx-WBS-SPECT/CT mainly involved lymph nodes of the central compartment with a corresponding negative ultrasound of the neck. The optimized basal-Tg cut-off was settled at 0.39 ng/mL by ROC curve analysis (AUC = 0.852) aiming to discriminate patients with positive or negative (123)I-Dx-WBS-SPECT/CT, respectively. Basal-Tg exceeding this cutoff level independently predicts a positive (123)I-Dx-WBS-SPECT/CT. In conclusion, (123)I-WBS-SPECT/CT identified lymph node metastases in 14/37 patients with less than ER and negative neck ultrasound, thus modifying the management of such patients. The diagnostic performance of (123)I-Dx-WBS-SPECT/CT imaging significantly increases in patients with basal-Tg levels ≥ 0.39 ng/mL. ABSTRACT: Background: In differentiated thyroid cancer (DTC) patients, the response to initial treatments is evaluated 6–12 months after radioiodine therapy (RIT) according to the 2015 American Thyroid Association (2015 ATA) criteria. In selected patients, diagnostic 131-radioiodine whole-body scintigraphy (Dx-WBS) is recommended. We evaluated the diagnostic performance of (123)I-Dx-WBS-SPECT/CT imaging in detecting incomplete structural responses in the early follow-up of DTC patients and, additionally, derived optimized basal-Tg value as a yardstick for scintigraphic imaging. Methods: We reviewed the records of 124 low or intermediate-risk DTC patients with negative anti-thyroglobulin antibody. All patients had undergone (near)-total-thyroidectomy followed by RIT. The response to initial treatments was evaluated 6–12 months after RIT. Results: According to the 2015 ATA criteria, 87, 19 and 18 DTC patients were classified to have excellent response (ER), indeterminate/incomplete biochemical response (BIndR/BIR) or structural incomplete response (SIR), respectively. Among patients with less than ER, 18 had a positive (123)I-Dx-WBS-SPECT/CT. Metastatic disease at (123)I-Dx-WBS-SPECT/CT mainly involved lymph nodes within the central compartment, and corresponding neck ultrasound examinations were negative. The ROC curve analysis was performed to define the best basal-Tg cut-off (i.e., 0.39 ng/mL; AUC = 0.852) able to discriminate patients with and without positive (123)I-Dx-WBS-SPECT/CT, respectively. The overall sensitivity, specificity, accuracy, PPV and NPV were 77.8%, 89.6%, 87.9%, 56.0% and 95.9%, respectively. Basal-Tg cut-off was an independent risk factor for having a positive (123)I-Dx-WBS-SPECT/CT. Conclusion: (123)I-Dx-WBS-SPECT/CT identified lymph node metastases in 14/37 patients with less than ER and a negative neck ultrasound, thus modifying the management of such patients. The diagnostic performance of (123)I-Dx-WBS-SPECT/CT significantly increased in patients with basal-Tg values ≥ 0.39 ng/mL.