Comparison between ^99mTcO₄⁻ SPECT/CT and diagnostic ¹³¹I SPECT/CT for the detection of postoperative thyroid remnant in patients with differentiated thyroid cancer

Background: Accurate imaging evaluation of remnant thyroid tissue (RTT) is particularly important for precise radioiodine remnant ablation (RRA). However, the optimal imaging method to guide its management remains a subject of debate. This study compared the diagnostic value of ^99mTcO₄⁻ single photon emission computed tomography/computed tomography (SPECT/CT; Tc-99m scan) and diagnostic 74 MBq ¹³¹I whole body scan with SPECT/CT (Dx scan) for detecting RTT in differentiated thyroid cancer patients after total thyroidectomy, to guide subsequent RRA strategy.

Methods: A total of 505 patients undergoing radioactive iodine (RAI) therapy were enrolled and had no cervical lymph node metastases or distant metastasis in postablation ¹³¹I SPECT/CT (Rx scan). The thyroid bed area was divided into 8 regions on SPECT/CT. RTTs on Tc-99m scan and Dx scan were counted and analyzed. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of Tc-99m scan and Dx scan for detecting RTT were compared. Associations of serum stimulated thyroglobulin (sTg), thyroglobulin antibody (TgAb), and sTg/thyroid-stimulating hormone (TSH) with findings across all scan modalities were analyzed. The ablation success rates in different groups were assessed.

Results: Per-patient analysis showed that Dx scan had significantly higher sensitivity (95.3%) and NPV (34.3%) than Tc-99m scan (66.1% and 6.7%, respectively). Both modalities achieved 100% specificity and PPV. Rx SPECT/CT detected 1,057 radioiodine-uptake foci in 493 patients, with the majority of RTT situated in the superior anterior region (SAR) and posterolateral regions. In the per-site analysis, the overall sensitivity, specificity, PPV, and NPV of Tc-99m scan for detecting RTT were 47.3%, 100%, 100%, and 84.3%, respectively, compared with 86.7%, 100%, 100%, and 95.5% for Dx scan. sTg levels were significantly higher in both Tc-99m scan-positive and Dx scan-positive patients than those in negative patients (2.12 vs. 0.33 ng/mL, P<0.001; 1.43 vs. 0.08 ng/mL, P<0.001). SAR-positive patients had significantly higher median sTg than SAR-negative patients (1.96 vs. 0.61 ng/mL, P<0.001). The ablation success rate was significantly higher in the 100 mCi group than it was in the 50 mCi group (97.7% vs. 90.0%, P=0.021). For SAR-positive patients, 100 mCi was superior to 50 mCi (96.8% vs. 88.2%, P=0.044), confirmed after propensity score matching [odds ratio (OR) =9.098, P=0.037]. No dose difference was seen in SAR-negative patients.

Conclusions: The Dx scan is superior to the Tc-99m scan for RTT assessment. To optimize RAI management, a Dx scan using 74 MBq ¹³¹I may be recommended before the first individualized RAI therapy.