The average size of a normal parathyroid gland is 5×3×1 mm; normal glands weigh between 40 and 50 mg. They are thus infrequently identified at imaging. Adenomas, on the other hand, are considerably larger, and have a mean mass of greater than 10 times of the normal parathyroid gland, and are thus often identified at cross-sectional imaging (9
). Ultrasonography and 99mTc-sestamibi scintigraphy were the dominant imaging techniques for preoperative localization of parathyroid adenomas. Numerous studies comparing these techniques suggest similar sensitivities and specificities for solitary adenoma detection (10
). Localization accuracy is also improved when both studies are obtained preoperatively (12
). Reported sensitivities for the detection of solitary parathyroid adenomas with preoperative ultrasonography range from 72% to 89% in recent large series (23
). A meta-analysis performed by Ruda et al (21
). encompassing 54 studies performed between 1995 and 2003 using ultrasonography for preoperative localization in primary hyperparathyroidism calculated ultrasonographic sensitivity for the detection of solitary adenoma, hyperplasia, and double adenoma to be 79% (95% confidence interval 77–80%), 35% (95% confidence interval, 30–40%), and 16% (95% confidence interval, 4–28%), respectively.
Sestamibi with 99mTc is the most commonly used radiotracer for imaging the hyperfunctioning parathyroid glands and has been extensively studied in the setting of primary hyperparathyroidism. Sestamibi is taken up by both the thyroid and parathyroid glands, but adenomatous and hyperplastic parathyroid tissue shows more avid uptake of the radiotracer and often retains the radiotracer longer than adjacent thyroid tissue. Thus, initial planar images obtained shortly after the administration of radiotracer will show both thyroid and parathyroid tissue (12
). Asymmetric foci of increased radiotracer uptake on early images can be seen, representing abnormal parathyroid tissue superimposed on the normal thyroid (14
). Delayed images, obtained approximately 2 hours after radiotracer administration, are acquired to look for foci of retained radiotracer characteristic of hyperfunctioning parathyroid tissue (15
A preoperative approach that combines both the anatomic information of sonography and the physiologic information of scintigraphy has been shown to predict the presence and location of solitary adenomas more accurately than either technique alone (17
). Lumachi (et al). retrospectively reviewed preoperative sonography and 99mTc-sestamibi findings in patients with proven solitary adenomas and found a combined sensitivity of 95% versus 80% for sonography and 87% for scintigraphy alone. Sonography has the advantage of being more specific regarding the site of an adenoma in relation to the thyroid gland (19
). Scintigraphy clearly has an advantage in the detection of ectopic glands, particularly in the mediastinum (20
). Given that the operation of choice for both multiglandular disease and double adenomas is a traditional bilateral approach, some endocrine surgeons have advocated that equivocal, negative, or discordant results on both preoperative studies warrant a nonselective approach because a high proportion of these patients will have multifocal disease (21
Our current large study of 80 unselected patients, confirms the validity of US for preoperative localization of parathyroid adenomas in patients with PHPT. Ultrasonography provided positive imaging results in 85% of these patients. The reliability of positive ultrasonographic imaging was high with 89.7% positive predictive value based on correlation with surgical findings. Overall, US correctly predicted the surgical findings in 76.3% of patients in which enlarged parathyroid glands were found at surgery. The sensitivity of US was 83.5% in this study. The ability of ultrasonography to correctly localize enlarged parathyroid glands in primary hyperparathyroidism ranged from 44-87% (5
), with the most recent studies reporting sensitivity of 67-87% in patients without prior parathyroid surgery (5
). Previously reported positive predictive values of 89-97% are also in concordance with the present results (8
). It is likely that the reported accuracy of US for preoperative localization of enlarged parathyroid glands is highly dependent on the skill and experience of the examiner (5
In the current study, we compared dual phase parathyroid scintigraphy with ultrasonography. The significant positive result reported by SS (86.25%) as well correctly predicting the surgical findings in 78.8% of patients with PHPT, signifies the importance of utilizing SS as well. The results however were not significantly higher than the corresponding value for US, (85% positive result, and predicting the surgical findings in 76.3% of patients). The sensitivity and positive predictive value were similar for US and SS based on correlation with surgical findings (83.5%, 89.7% vs. 85%, 91.3%).
Among previous reports that have directly compared US and SS in patients undergoing initial parathyroid surgery, Mazzeo et al (1996) (13
) and De feo et al (2000) (6
) reported that the two methods were similar in their ability to correctly predict the surgical findings, while Casas et al (1993) (14
) and Lumachi et al (2000) (1
) found that the SS imaging was superior. In a large study encompassing US in 449 patients and SS in 700 of these patients, Cha Puis et al (1996) (15
) found that the US provides better results.
We also determined whether the patients with more severe hypercalcemia, higher PTH levels, and larger abnormal parathyroid glands are more likely to have positive tests. The parameters in patients with localized adenomas detected by US and SS were compared with those undetected by localization studies. The only significant difference between the groups was mean intact preoperative PTH level which was higher in patients that their adenoma was detected by SS.
Taken together with the present study, it appears that there is a little overall difference between the ability of ultrasonography and scintigraphy to correctly localize abnormal glands in patients without prior surgery for PHPT.