Parathyroid Imaging: How Good Is It and How Should It Be Done?

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Hypersecretion of parathormone in primary hyperparathyroidism is common, occurring in approximately 1 in 500 women and 1 in 2,000 men per year in their fifth to seventh decades of life. This has been suggested from the literature to be primarily the result of a parathyroid adenoma (80-85% of cases), hyperplasia involving more than 1 gland, usually with all 4 glands being involved (10-15% of cases), or the result, albeit rarely, of parathyroid carcinoma (0.5-1% of cases). Surgical removal of the hypersecreting gland is the primary treatment; this procedure is best performed by a skilled surgeon who would normally find the abnormality in 95% of cases. Imaging, however, should be used to identify the site of abnormality, potentially reducing inpatient stay and improving the patient experience. Functional imaging of parathyroid tissue using thallium was introduced in the 1980s but has largely been superceded by the use of 99mTc-labeled isonitriles. The optimum techniques have used 99mTc-sestamibi with subtraction imaging or washout imaging. A recent systematic review reported the percentage sensitivity (95% confidence intervals) for sestamibi in the identification of solitary adenomas as 88.44 (87.48-89.40), multigland hyperplasia 44.46 (41.13-47.8), double adenomas 29.95 (−2.19 to 62.09), and carcinoma 33 (33). This review does not separate the washout and subtraction techniques. The subtraction technique using 99mTc-sestamibi and 123I is the optimal technique enabling the site to be related to the thyroid tissue when the parathyroid gland is in the neck in a normal position. If there is an equivocal scan then confirmation with high resolution ultrasound should be used. With ectopic glands, the combined use of single-photon emission computed tomography may then provide anatomical information to enable localization of the functional abnormality. In patients who have had surgical exploration by an experienced parathyroid surgeon in a unit with an experienced nuclear medicine team and negative sestamibi imaging, it is reasonable to image the patient with 11C methionine. It is debatable whether patients with a high likelihood of secondary hyperparathyroidism should be imaged. The only possible justification for this is to exclude an ectopic site. There is no substitute for an experienced surgeon and an experienced imaging unit to provide a parathyroid service.

Section snippets

Sites of Parathyroid Glands

There can be 2 to 6 parathyroid glands (normally 4). The normal sites are posterior to the thyroid, related to the upper and lower poles of the right and left lobes. A small number of patients, approximately 5%, have more than 4 glands, and a further 5% have only 3 glands.2, 3 The parathyroid glands may be found in a number of ectopic sites in the neck or upper mediastinum. The upper glands may be found posterior to the esophagus or occasionally in the carotid sheath, and the lower glands may

Preoperative Localizing Techniques

There are a variety of anatomical and functional methods for localizing abnormal parathyroid tissue that have developed over a number of years and have been improved on with the technological developments occurring over the same time period. Functional techniques have included 75Se-selenomethionine, 57Co-vitamin B12, 131I-toluidine blue, 123I-methylene blue, which have been used with little success5 although the use of methylene blue or toluidine blue peroperatively, without radiolabeling,

Imaging Methodology

Ideally, we would use a tracer that was specific to the parathyroid alone and provide an anatomic image to localize it. Unfortunately, at the present time no such tracer exists and, therefore, a number of methods have been used, each purporting to have advantages over other techniques.

Conclusion

The recent summary statement by a consensus panel on asymptomatic primary hyperparthyroidism states “preoperative localization testing is mandatory when the MIP procedure is used. Preoperative localization tests should not be used to make, confirm, or exclude the diagnosis of primary hyperparathyroidism.”1 It also concludes that the key elements are an experienced parathyroid surgeon and an experienced imaging unit result in the highest success in identifying and removing abnormal parathyroid

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