Parathyroid adenoma with rapid washout on a dual-phase sestamibi scan
A 65-year-old male was referred to the endocrine clinic for the evaluation of hypercalcemia and hyperparathyroidism after atypical dual-phase sestamibi parathyroid scan. He had a 10-year history of recurrent nephrolithiasis complicated by a partially obstructing proximal right ureteral calculus.
His laboratory tests showed calcium 11.3 mg/dL, parathyroid hormone (PTH) 257 pg/mL (normal: 10-60 pg/mL), phosphorus 2.1 mg/dL (normal: 2.3-3.7) and 25-hydroxyvitamin D 47 ng/mL (optimal: 30-60 ng/mL). His bone mineral density was normal with a lumbar spine density (L2-L4) of 1.139 g/cm2 (T-score: –0.76) and femoral neck density of 1.47 g/cm2 (T-score: –0.92). Compared with his bone density measurement 14 months prior, there was an insignificant 0.5% decrease in the lumbar spine and a significant 5.4% decrease in the femoral neck.
Thyroid, parathyroid scans
A dual-phase Tc-99 sestamibi PTH scan and Tc-99m pertechnetate thyroid scan were ordered. After an IV injection of Tc-99 sestamibi, an immediate anterior view of the neck and chest was performed (Figure 1A) that showed isotope trapping in the thyroid and an elongated area inferior to the left thyroid lobe.
After 2 hours, delayed views showed the expected washout of isotope from the thyroid and the area extending from the left thyroid lobe (Figure 1B). Because of the atypical fast washout from the suspected PTH adenoma, a Tc-99m thyroid scan was performed that showed uniform, symmetrical trapping of isotope in the thyroid but not in the area inferior to the left thyroid lobe.
The patient was referred for an ultrasound to confirm the location of the PTH adenoma before surgery. Our office ultrasound showed a normal-sized thyroid with several subcentimeter spongiform nodules. Inferior and separate from the inferior pole of the left thyroid lobe was a hypoechoic mass that measured 2.6 cm x 0.8 cm x 0.6 cm (sagittal x depth x transverse). The mass was vascular by Doppler analysis, but a feeding vessel could not be identified (Figure 2). The mass was low and located posterior to the clavicular head. Because the mass did not have a typical feeding vessel and an atypical rapid washout on the sestamibi scan, a single ultrasound-guided aspiration of the mass was performed. The cytology contained insufficient cells for diagnosis but the PTH level on the needle wash was elevated at >2,500 pg/mL. A focused surgery removed a 1.4 g left inferior parathyroid adenoma. His intraoperative PTH level fell from 332 pg/mL at baseline to 116 pg/mL at 10 minutes and 4 pg/mL at 30 minutes after the removal of the parathyroid adenoma.
Approximately 84% of PTH adenomas are single and in normal positions just posterior or inferior to the thyroid. PTH adenomas are usually well-circumscribed ovoid, solid and homogenously hypoechoic. Management of primary hyperparathyroidism has evolved. Preoperative localization by ultrasound or nuclear imaging, along with rapid intraoperative PTH assay allows focused and minimally invasive surgery. In the dual-phase nuclear medicine imaging, washout over time of sestamibi from the thyroid is more rapid than a PTH adenoma. This differential washout allows localization of parathyroid adenomas by persistent uptake in the PTH adenoma on the delayed scan. Sestamibi scanning has a sensitivity of 54% to 100%, with most series in the 80% or 90% range. False-negative sestamibi scans may occur because of rapid tracer washout; in this patient, small focal lesions were detected upon image review after surgery and nodular thyroid disease.
Ultrasound is a sensitive method for detecting PTH adenomas without the radiation exposure. The sensitivity of ultrasound for detecting enlarged PTH glands ranges from 70% to 100%. False-positive or negative sonographic diagnosis may be due to thyroid nodules, prominent blood vessels, lymph nodes, esophagus and longus colli muscle. Many studies comparing the sensitivity of ultrasound to sestamibi scanning show no significant difference in the sensitivity to detect PTH adenomas. A study of 516 patients demonstrated that ultrasound localized PTH adenomas in 87% of patients, whereas sestamibi correctly identified the location in 76% (P<.001). Further, if the ultrasound localization was performed first, sestamibi scans provided additional information in only an additional 8% of patients. This study suggested that ultrasound localization of a single PTH adenoma is more accurate than dual-phase sestamibi imaging and should be the initial study for the pre-surgical localization of a PTH adenoma. It is helpful to perform both imaging techniques if the initial test is non-localizing, as the positive predictive value to locate a single PTH adenoma after performing both scans is 90%.