Published data show marked variability of Thallium/pertechnetate scintigraphy results. Regrouping 14 studies by Hauty et al. with a total of 317 patients, the accumulated sensitivity to detect parathyroid adenomas was 82%. The diagnostic accuracy was 78%, the positive predictive value 94% and the false positive rate was 5%.
Although dual isotope parathyroid scintigraphy has shown satisfactory results, it suffers from technical limitations and interpretative pitfalls. These include: unfavorable Tl-201 dosimetry, 80 keV photons of thallium, poor quality images, rigorous technique, prolonged patient immobilization and poor thyroid uptake of pertechnetate due to interfering medication or recent iodinated contrast media. Improved parathyroid scintigraphy has been reported with Tc-99m Sestamibi.
Tc-99m Sestamibi offers several technical and interpretative advantages over Thallium. Like Thallium, MIBI accumulates in both the thyroid and parathyroid glands with a peak activity at 4-6 minutes after injection. Both tracers "wash out" from the thyroid gland quickly; yet unlike Thallium, MIBI is retained in abnormal parathyroid, facilitating visualization.
The mean uptake of Thallium is greater than that of MIBI in hyperplastic, adenomatous parathyroid and thyroid tissue. There is however no difference in uptake of Thallium between parathyroid adenoma and thyroid tissue, whereas there is a clear difference in uptake of MIBI in these tissues. This difference would facilitate visualization of pathology due to a higher target to background ratio. The literature suggests that the maximum difference between MIBI uptake in the thyroid and the parathyroid occurs between 15-28 min.
The degree of MIBI avidity by the parathyroid adenoma is independent of PTH levels, but depends, in part, on gland size. The smallest reported pathologic gland weighed 150 mg; the largest weighed 8 gm. For yet unclear reasons, large (2 gm) adenomas may not accumulate sufficient MIBI for detection. The mechanism of MIBI localization in adenomas is not entirely clear. Chiu et al. have suggested that Tc-99m-MIBI is sequestered within the cytoplasm and mitochondria in response to electrical potentials generated across the membrane bilayers of both the cell and the mitochondria. This suggests that a tissue with a large number of mitochondria (e.g. myocardial cell) may take up MIBI more avidly and retain the tracer longer. Recently, Sandrock et al. showed that the parathyroid adenomas had a large number of mitochondria per cell.
To date approximately 450 patients with hyperparathyroidism have been studied with Sestamibi. The sensitivity ranges from 88-100%. In studies comparing MIBI to Tl-201, MIBI has either shown a parathyroid adenoma more convincingly or localized one that was missed with Tl-201. No a single adenoma positive with Tl-201 and negative with MIBI has been reported.
2. Taillefer R, Boucher Y, Potvin C, Lambert R. Detection and Localization of Parathyroid Adenomas in Patients with Hyperparathyroidism Using a Single Radionuclide Imaging Procedure with Technetium-99m-Sestamibi (Double-Phase Study). JNM 1992; 33: 1801-1809.
3. Oates Elizabeth. Improved parathyroid scintigraphy with Tc-99m MIBI a superior radiotracer. App Rad 1994: 37-40.
4. Chiu ML, Kronauge JF and Piwnica-Worms D. Effect of Mitochondrial and Plasma Membrane Potentials on Accumulation of Hexakis (2-Methoxyisobutylisonitrile) Technetium in Cultured Mouse Fibroblasts. JNM 1990; 31:1646-1653.
5. Goris ML, Basso LV, Keeling C. Parathyroid Imaging. JNM 1991; 32: 887-889.
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