The long-axis lengths and calculated volumes of uptake-positive lesions were significantly larger than those of uptake-negative lesions in MET PET/CT (unpaired t-test for length and the Mann–Whitney U-test for calculated volume). No significant difference was found in intact PTH levels between MET PET/CT-positive and MET PET/CT-negative patients (Mann–Whitney U-test) (Table 4).
The same analysis was performed on MIBI SPECT/CT results. Similar to the results of MET PET/CT, uptake-positive lesions were larger than uptake-negative lesions. No significant difference was observed in intact PTH levels between MIBI SPECT/CT uptake-positive and MIBI SPECT/CT uptake-negative patients (Table 5).
MIBI SPECT/CT did not detect additional lesions compared with MIBI SPECT or planar imaging in any case, although MIBI SPECT/CT allowed more detailed localization in one patient with an ectopic parathyroid gland. The comparison between MIBI SPECT/CT and SPECT is consistent with Gayed’s report demonstrating that MIBI SPECT/CT had no additional value over conventional MIBI SPECT imaging except for localization of ectopic parathyroid glands 12. In contrast, Kim et al. 4 found higher sensitivity of MIBI SPECT/CT compared with MIBI SPECT, dual-phase MIBI scintigraphy, and conventional imaging. Although it is unclear whether SPECT/CT has higher sensitivity than SPECT, SPECT/CT may be recommended when an ectopic parathyroid gland is suspected. In addition, SPECT/CT has advantages for patients who have undergone previous thyroid or parathyroid surgery, who have multiglandular disease, or have nodular thyroid disease, and it improves interobserver agreement 3,13. Our results are inconsistent with previous reports showing higher sensitivities of SPECT or SPECT/CT compared with planar imaging 1,2,14,15. Although the reason for this is unclear, SPECT is theoretically considered to have an advantage over planar imaging by providing detailed information concerning a three-dimensional location 14.
Five patients showed MET-positive/MIBI-negative results and four patients showed MET-negative/MIBI-positive results, indicating these two modalities play a complementary role. MET PET/CT showed positive results in 56% (5/9) of patients who had shown negative results in MIBI SPECT/CT. Therefore, MET PET/CT would be a feasible option if MIBI-negative results are obtained in localization of adenomas/hyperplasia. Such complementary results were also observed in a previous report 10. Incorporation into PTH and its precursors is considered to be one of the mechanisms of MET uptake into adenomas/hyperplasia, whereas MIBI uptake is considered to be positively related to increased concentrations of mitochondria-rich oxyphil cells and negatively related to the expression of p-glycoprotein, or multidrug resistance-related protein 16. Such different mechanisms of tracer uptake may account for their complementary role in localizing parathyroid gland adenomas/hyperplasia.
In the present study the positive lesions in both modalities were larger than the negative ones. The correlation between lesion detectability in both modalities and lesion size or weight was observed in previous reports 17–19. Only three (38%) and one (13%) of eight lesions less than 276 μl could be detected by MET PET/CT and MIBI SPECT/CT, respectively. In contrast, both modalities depicted nine (90%) of 10 lesions equal to or larger than 276 μl.
One possible factor affecting MET and MIBI uptake is the intact PTH level. There is a previous report showing a positive correlation between MET uptake of the lesion and serum intact PTH level 11. Per-patient sensitivities of both modalities were 100% in six patients with intact PTH levels greater than 150 pg/ml in our population. However, no significant difference in intact PTH levels was observed between positive and negative cases in the two modalities, which is consistent with a previous report by Weber et al. 6.
Dual-tracer parathyroid scintigraphy using a pinhole collimator has been demonstrated as facilitating more accurate localization of parathyroid lesions compared with a single-tracer, dual-phase protocol with or without pinhole collimator or SPECT 20–22. In particular, accurate localization of parathyroid lesions using this technique has been shown in cases of multiple parathyroid gland disease 23, for which the sensitivities of both MET PET/CT and MIBI SPECT/CT were insufficient in the present study. Therefore, this technique might have detected more parathyroid lesions if it had been performed. A comparison between dual-tracer scintigraphy and MET PET or PET/CT has not been fully carried out, although Schalin-Jäntti et al. 8 reported comparable accuracy of these techniques in the reoperative setting of primary hyperparathyroidism.
There are several limitations in the present study. First, histological confirmation was not obtained for all lesions. Therefore, it is possible that positive findings in some patients who did not undergo surgery were false positive. However, we believe that the possibility for false-positive cases is low, because suspected enlarged parathyroid glands were detected by ultrasonography corresponding to MET or MIBI uptakes in such patients. Second, a relatively small number of patients were analyzed in this investigation. Finally, we used only delayed SPECT/CT without obtaining early SPECT/CT. There is one article demonstrating higher sensitivity of early SPECT/CT 14, and different results might have been acquired with utilization of early SPECT/CT.
There are no conflicts of interest.
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