The use of non-specific tracers in the follow up of differentiated thyroid cancer: results with Tc-99m tetrofosmin whole body scintigraphy.

Since many years TI-201, as non-specific tumor-searching radionuclide, plays a certain although somewhat controversial role in the follow-up of differentiated thyroid carcinoma (DTC). Recently some Tc-99m labeled myocardial perfusion agents were introduced, that might be more important for nuclear oncology in the future. Aim of this study was to evaluate prospectively the reliability of the new non-specific tumor searching tracer tetrofosmin (Myoview) in the post operative follow-up of differentiated thyroid carcinoma during TSH suppressive thyroid hormone treatment and to compare the results in patients with metastasizing DTC to Tc-99m sestamibi (Cardiolite) and TI-201. In a pilot study 12 patients with elevated thyroglobulin (Tg) levels of more than 10 ng/ml and known metastatic disease were examined under TSH suppressive L-Thyroxine treatment comparing TI-201, Tc-99m sestamibi and Tc-99m terofosmin whole body scintigraphy (WBS). Furthermore in 146 consecutive follow up patients tetrofosmin WBS was performed under TSH-suppressive L-T4 treatment. The results were compared to serum thyroglobulin (Tg), ultrasonography (US) of the neck, I-131 whole body scintigraphy (I-131 WBS), transmission computed tomography (TCT) or magnetic resonance imaging (MRI) and bone scintigraphy. Whole body scans were performed with TI-201 (74 MBq; 20 min post injection), Tc-99m sestamibi (370 MBq; 20-60 min post injection) and Tc-99m tetrofosmin (370 MBq; 20-60 min post injection). Tumor/background ratios and optional time/activity analyses (up to 120 min post injection) were evaluated using the region of interest approach. In the pilot study tetrofosmin showed the highest T/BG ratios and detection rates (T/BG: 1.76 +/- 0.345) followed by TI-201 (T/BG: 1.59 +/- 0.396) and sestamibi (1.51 +/- 0.31 p = 0.05). From the 146 patients investigated consecutively with Tc-99m tetrofosmin WBS for the routine follow up of DTC, 88 patients (no thyroid remnants, no history of metastases or tumor recurrence) were tumor free. All of them resulted in negative Tc-99m tetrofosmin WBS. Another 32 patients (papillary carcinoma pT1) were also in complete remission, but had sonographically proven remnants (echonormal). Twenty one of them exhibited certain Tc-99m tetrofosmin accumulation in the thyroid bed. In 9 cases with local recurrence as confirmed by histopathology after reoperation or by cytology after fine needle aspiration, the tetrofosmin scintigraphy clearly revealed relapse of malignancy including 2 patients with tetrofosmin positive additional distant metastases. Seventeen patients had distant metastases (11 pulmonary, 3 bone, 2 bone and pulmonary, 1 bone and soft tissue) detected by different modalities and resulting in a total of 44 lesions to be evaluated. In the 23 radioiodine negative metastases, 17 were also detected by tetrofosmin (74%). In the 21 radioiodine accumulating lesions 19 were Tc-99m tetrofosmin positive (90%). Four cases with radioiodine negative disseminated lung metastases showed diffuse pulmonary tetrofosmin uptake. This prospective study shows that Tc-99m tetrofosmin is a new promising tracer to detect malignant recurrence and distant metastases in the follow up of DTC without the necessity of thyroid hormone withdrawl, especially in patients with elevated Tg level and no iodine uptake. Tc-99m tetrofosmin shows slight advantages concerning T/Bg Ratio, background clearance, detection rate and dosimetry compared with TI-201 and Tc-99m sestamibi.