Incremental diagnostic value of preoperative ^99mTc-MIBI SPECT in patients with a parathyroid adenoma

The purpose of this prospective study was to evaluate the diagnostic value of early parathyroid SPECT combined with quantitative analysis as compared with planar imaging in patients undergoing minimally invasive radioguided surgery. Methods: A total of 52 consecutive patients with primary hyperparathyroidism underwent planar and SPECT parathyroid scintigraphy 2-5 d before surgery. Each patient had a single-tracer dual-phase technique using ^99mTc- methoxyisobutylisonitrile (^99mTc-MIBI) and a double-tracer subtraction technique using a delayed ^99mTc-pertechnetate scan. Immediately after the first ^99mTc-MIBI planar image, a SPECT study was acquired. Before radioguided parathyroidectomy, each patient was reinjected with ^99mTc-MIBI. Serum calcium levels were available for all patents before surgery and at 8 and 24 h after surgery. Serum parathyroid hormone (PTH) levels were also available for all patients. Quantitative analysis was performed using the average count ratio of parathyroid to left thyroid lobe, right thyroid lobe, and maximum thyroid activity. All patients had histopathologic examination of the removed glands. Results: The average time for radioguided surgery was 30 min (range, 20-40 min). Post-surgical calcium levels correlated significantly with the adenoma weight (r = 0.5; P = 0.016). Combined planar scintigraphy correctly identified 41 adenomas (79%). SPECT increased the sensitivity to 96%. SPECT was superior to planar imaging in 9 patients, mainly in patients with ectopic adenomas or with multinodular goiters. Gland size did not affect significantly the detectability of SPECT. ^99mTc-MIBI retention was noted in only 31 adenomas (60%). The average uptake ratios of parathyroid counts to the left lobe, right lobe, and maximum thyroid activity were 1.20 ± 0.42, 1.29 ± 0.45, and 0.84 ± 0.35, respectively. The latter ratio was significantly correlated with PTH levels before surgery (r = 0.408; P = 0.04). Conclusion: Our data indicate that early preoperative SPECT in patients with primary hyperparathyroidism is essential for accurate localization of parathyroid adenomas and for the selection of patients who are candidates for minimally invasive radioguided surgery. Planar parathyroid imaging is less sensitive compared with SPECT, and washout kinetics of ^99mTc-MIBI are unreliable in the dual-phase technique. Patients with higher presurgical PTH levels may especially benefit from radioguided surgery.