131I-SPECT/CT at the first radioiodine ablation and during follow-up in patients with differentiated thyroid carcinoma (DTC).

Aim/Introduction: 131I-SPECT/CT has dramatically improved whole body scan (WBS) image interpretation in DTC patients after thyroidectomy. We further investigated SPECT/CT after frst radioiodine ablation and during long-term follow-up to obtain the best management of afected DTC patients. Materials and Methods: We retrospectively evaluated 106 consecutive thyroidectomized DTC patients at primary radioiodine ablation; 24 patients were at high risk (H), 61 at low risk (L) and 21 at very low risk (VL). Both WBS and SPECT/CT were performed 5-7 days after 1.85-5.66 GBq oral therapeutic dose using hybrid dual-head gamma camera with high energy, parallel hole collimators. Eight-six patients could be monitored in the follow-up repeating both WBS and SPECT/CT after radioiodine diagnostic dose (185 MBq) together with thyroglobulin assay. Results: SPECT/CT concordantly with WBS detected 172 residues, only it characterizing other 36 residues unclear (n.24) or occult (n.12) at WBS. Moreover, SPECT/CT correctly classifed 49 malignant foci in 17/106 patients (8H, 7L, 2VL) with signifcant (p<0.001) more elevated number than WBS which evidenced 32/49 foci in 13/17 patients. WBS classifed as unclear 17/32 foci and wrongly classifed other 2/32 foci correctly classifed as metastases by SPECT/CT which also characterized 17/49 further malignant foci occult at WBS. SPECT/CT had an incremental value over WBS in 25.5% of the 106 patients and changed classifcation and therapeutic management in 16.03% of cases. Only SPECT/CT also changed neck lymph node and distant metastasis classifcation performed at surgery in 11 cases. The 86 patients monitored during follow-up also included 13 of 17 patients with metastases at post-therapeutic scans. Four/13 patients underwent disease progression with metastasis number increase as ascertained by diagnostic SPECT/CT and with persistently high thyroglobulin levels; other 4/13 patients had stable disease with unmodifed metastatic lesions and thyroglobulin levels, while the remaining 5/13 patients showed disease improvement with reduction or absence of metastases and signifcant decrease of thyroglobulin levels. Moreover, other 13/86 patients (4H, 6L, 3VL) with only residues at posttherapeutic scan, showed 16 metastatic lesions in the follow-up, 13 unclear and 8 occult at WBS, only characterized by SPECT/ CT; thyroglobulin levels were undetectable or very low in 5/13 patients, two of whom VL-T1aN0M0, while these increased in the remaining 8/13 cases. Conclusion: 131I-SPECT/CT proved higher performance than WBS in both post-radioiodine therapy and follow-up phases of DTC patients to establish correctly stage and risk stratifcation and to evaluate disease progression and regression. Routine SPECT/CT use is suggested in DTC protocol