The role of routine 131I-SPECT/CT use in differentiated thyroid carcinoma (DTC) follow-up

Background-Aim: 131I-SPECT/CT, obtaining functional and anatomic fusion images, has proved capable of improving the performance of planar whole-body scan (WBS), for many years considered the reference diagnostic tool in patients thyroidectomized for DTC in association with serum Tg assay and radiological procedures. We further investigated whether routine SPECT/CT use may have an incremental value than WBS increasing sensitivity and accuracy and allowing precise anatomic localization and characterization of the lesions. Methods: We retrospectively evaluated 880 consecutive DTC patients during the follow-up after total thyroidectomy and radioiodine ablation; 801 patients had papillary carcinoma, 45 follicular carcinomas, and 34 Hu¨rthle cell carcinoma. Carcinomas sizes were [10 mm in 521 cases and B 10 mm in 359 cases. At surgery, according to risk stratification, 203 patients were at high risk (H), 454 at low risk (L), and 223 at very low risk (VL). In all patients, 48/72 h after radioiodine oral dose (185 MBq) both WBS and SPECT/CT were routinely performed using a variable angle hybrid dual-head gamma camera (Infinia Hawkeye, GE) with high energy, parallel-hole collimators. SPECT images were reconstructed with OSEM iterative method and fused with CT using a dedicated package (Xeleris, GE). Results: Globally, SPECT/CT correctly classified as malignant 362 foci, 263 in the neck and 99 outside the neck, in 192/880 patients, 52 of whom with papillary microcarcinoma, with a significant (p\ 0.0001) more elevated number than WBS which detected 121 malignant foci, 72 in the neck and 49 outside the neck, in 72/880 patients, all evidenced by SPECT/CT. SPECT/CT identified 85 single metastatic lymph nodes in the neck, 68 of which occult at WBS and 5 single distant metastases (1 lung, 1 mediastinum, and 3 bones) all of these unclear or occult at WBS; 10 patients with neck single metastases occult at WBS had undetectable Tg levels, were T1aN0M0 and 5 of them VL, and one patient L with lung metastasis had low Tg value (\2.5 ng/ml). SPECT/CT changed lymph node and distant metastasis classification performed at the surgery in 154 patients, from N0 to N1 in 117 cases, from M0 to M1 in 23 cases, and from N0M0 to N1M1 in 14 cases. SPECT/CT had an incremental value than WBS in 62% of positive patients and changed classification and therapeutic management in 45.5% of cases. SPECT/CT also characterized 41 benign or physiologic foci, unclear at WBS, 18 representing single foci, thus avoiding unnecessary treatments, with the reduction of false-positive results at WBS. Conclusions: SPECT/CT proved highly accurate in localizing and characterizing metastases from DTC during follow-up also modifying patient classification defined by WBS and allowing the correct change in therapeutic management. The procedure was also able to reduce the number of unclear WBS findings showing an incremental value, particularly when WBS is inconclusive and Tg levels are undetectable or very low in L and VL cases. Wider routine use of SPECT/ CT is suggested in DTC follow up contributing to reassessing the current management protocol.