Parathyroid Adenoma

Hyperparathyroidism is a common disorder, estimated to affect 1 in 500 women and 1 in 2000 men over the age of forty. Although most patients are asymptomatic, the clinical presentation has been summarized for medical students as “bones, stones and abdominal groans”, reflecting the multisystem effects of hypercalcemia. The specific diagnosis is verified by the finding of increased levels of serum parathormone. Primary hyperparathyroidism in 80-85 % of patients is caused by parathyroid adenoma, usually a single tumor. The great majority of the remaining cases are caused by parathyroid hyperplasia, with a very small number resulting from either multiple adenomas or parathyroid malignancy. Parathyroid adenoma has also been reported as an infrequent incidental finding in sonography exams performed for thyroid disease or for carotid artery stenosis [1, 2]. Parathyroid adenoma may involve any of the four parathyroid glands bordering the upper and lower poles of the thyroid gland. Ectopic parathyroid tissue is not uncommon, usually residing in the mediastinum, within the thyroid gland, or in other locations in the neck. Adenomas in these locations are problematical for diagnosis and treatment. The parathyroid glands are seen on imaging exams when they are enlarged; normal parathyroid glands are inapparent on imaging. When clinical and laboratory diagnosis indicates the likelihood of parathyroid adenoma, surgical exploration of the neck without preoperative imaging has been reported to have a success rate of 90-95% in tumor identification and resection. However, preoperative imaging may assist in localizing the tumor, finding additional adenomas, or suggesting an ectopic location if an adenoma is not found in its usual site. In patients with previous neck surgery, or other potential anatomic obstacles to routine surgical exploration, preoperative imaging becomes more significant [3]. Ultrasound is usually considered the best first-line imaging modality being universally available, inexpensive and not involving radiation exposure; however, there are proponents of scintigraphy being the preferred initial examination [4]. On sonography, a parathyroid adenoma is usually found bordering the thyroid gland, but sometimes in other locations in the neck, and uncommonly within the thyroid gland itself. Ectopic mediastinal adenoma is not accessible to sonography. The adenoma is usually an oval, circumscribed, hypoechoic, homogeneous structure on gray-scale imaging [5]. Color Doppler often shows flow within the tumor, at times eccentric in location. Unlike a lymph node, there is no hilus and no clear-cut hilar vessels. In addition, the Doppler finding of a vascular arc surrounding the gland in two thirds of cases has been described as helpful in differentiating adenoma from lymph node [6]. The sensitivity and specificity of ultrasound for parathyroid adenoma has been reported from 55-93 % and 40-98 % respectively, with limitations caused by ectopic glands, small adenomas, and confounding soft tissue structures in the neck. When sonography does not reveal an expected parathyroid adenoma, scintigraphy is the next line of imaging. Previous scintigraphic methods used subtraction of images based on the differential uptake between thyroid and abnormal parathyroid. The examination of choice today uses 99m Tc sestamibi as a sole imaging agent. Rather than differential uptake, this technique is based on differential washout of the radiopharmaceutical between normal thyroid (rapid washout) and abnormal parathyroid tissue (much delayed washout). Thus in a positive exam, an early image 10-15 minutes after injection (“thyroid phase”) shows generalized uptake in the region of the thyroid; however, on delayed images up to 3 hours post-injection (“parathyroid phase”), the 99m Tc sestamibi remains only within abnormal parathyroid tissue. The camera’s field of view should include the upper chest for discovery of ectopic parathyroid tissue outside the neck. In addition to planar imaging, SPECT imaging is also useful. Sensitivity for detection of parathyroid adenomas has been reported from 70-100 %, the major problem being smaller parathyroid adenomas. In difficult cases there is still a place for the two-isotope subtraction technique [7]. CT and MR examinations may be ordered for discovery of adenoma; alternatively, incidental findings on either exam may suggest parathyroid pathology. On contrast-enhanced CT scans, an enlarged, enhancing soft-tissue mass near the expected location of the parathyroid glands is consistent with parathyroid adenoma. Sensitivity of CT in discovery of parathyroid adenoma has been reported from 40-90 %. MR imaging of the neck may reveal a parathyroid adenoma which is typically low in signal on T1 weighted images, and bright on T2. However, this appearance is variable, and 30 % of abnormal parathyroid glands have atypical appearance due to hemorrhage or necrosis or fibrosis. Nonetheless, MR has been reported to have close to 90 % accuracy in the search for ectopic parathyroid adenoma. Treatment of parathyroid adenoma is surgical excision, with minimally invasive techniques becoming more widespread. Some techniques use intraoperative localization with injected 99m Tc sestamibi to precisely guide the surgeon to the adenoma.