Use of Tc-sestamibi SPECT/CT when conventional imaging studies are negative for localizing suspected recurrence in differentiated thyroid cancer: a method and a lesson for clinical management.

Purpose: The detection of recurrent disease in differentiated thyroid cancer (DTC) patients with elevated or rising serum thyroglobulin (Tg) levels and multiple negative conventional imaging studies can be challenging, especially when F-FDG PET/CT scan is also negative. We report a patient and review the literature on the diagnostic use of Tc-sestamibi scans to identify the source of elevated or rising Tg in patients with negative conventional imaging including negative F-FDG PET/CT scans.

Patient And Methods: A 73-year-old woman was referred for widely-invasive metastatic follicular thyroid cancer with bone metastasis to her left mandible. She had a total thyroidectomy, left mandibular resection, and I therapy of 145 mCi (5.4 GBq) and her subsequent unstimulated serum Tg level was 29 ng/ml (TgAb negative). At six months' follow-up, her stimulated Tg was 527 ng/ml (TSH 188 mIU/L, TgAb negative). All imaging studies performed within the prior 12 months were reported as negative for recurrence or metastasis; this included neck ultrasound, diagnostic radioiodine scan, chest CT and, F-FDG PET/CT. The patient was injected with 24.6 mCi (910 MBq) of Tc-sestamibi intravenously, and whole-body and SPECT/CT images were acquired.

Results: The Tc-sestamibi whole-body posterior image demonstrated abnormal focal uptake in the right posterior calvarium and corresponded to an occipital lytic bone lesion on the SPECT/CT. The patient underwent surgical resection of the skull metastasis, and pathology confirmed metastatic follicular thyroid cancer. Five months post-surgery, the suppressed Tg was markedly reduced and remained stable at ~3.2 ng/ml. With the knowledge of the DTC recurrence location, the two sets of F-FDG images were re-evaluated. The more thorough and targeted interpretation underscored the importance of structured image reporting. The current literature on the utility of Tc-sestamibi scans when radioiodine, F-FDG PET/CT, and other imaging studies are negative is sparse and inconsistent.

Conclusions: Tc-sestamibi may have a role in thyroid cancer localization when physical exam, neck ultrasound, radioiodine scan, chest/abdomen CT, and F-FDG PET/CT does not identify the source of elevated Tg levels in DTC.