Use of 99m 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 18 F-FDG PET/CT scan is also negative. We report a patient and review the literature on the diagnostic use of 99m Tc-sestamibi scans to identify the source of elevated or rising Tg in patients with negative conventional imaging including negative 18 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 131 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, 18 F-FDG PET/CT. The patient was injected with 24.6 mCi (910 MBq) of 99m Tc-sestamibi intravenously, and whole-body and SPECT/CT images were acquired.

RESULTS: The 99m 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 18 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 99m Tc-sestamibi scans when radioiodine, 18 F-FDG PET/CT, and other imaging studies are negative is sparse and inconsistent.

CONCLUSIONS: 99m Tc-sestamibi may have a role in thyroid cancer localization when physical exam, neck ultrasound, radioiodine scan, chest/abdomen CT, and 18 F-FDG PET/CT does not identify the source of elevated Tg levels in DTC.