Sensitivity of 99m-Tc-MIBI parathyroid scintigraphy and USG is increased when combined with serum PTH and Ca values in patients with primary hyperparathyroidism

Objectives To establish a diagnostic cut-off for serum iPTH and Ca that can predict parathyroid scan (PS) and USG positivity and combine biochemical values with imaging results to improve the sensitivity of PS and USG for histopathologically confirmed parathyroid lesions with prior to surgery.

Methods The study population consisted of 313 patients (238 F, 75M; mean ± SD: 55 ± 15 yrs). Planar images and SPECT of the neck and chest were acquired at 20 minutes and 2 hours after intravenous injections of 20mCi of Tc-99m-(methoxyisobutylisonitrile) MIBI. USG was done to evaluate glandular structure and blood tests were performed to determine serum intact parathyroid hormone (PTH), calcium and phosphorus concentration. The optimal cut-off values were estimated from the ROC curves.

Results One hundred and twenty-two of the patients underwent surgery and a total of 116 parathyroid lesions were surgically excised (100 adenoma, 5 carcinoma, 11 hyperplasia). Mean serum PTH and calcium values were significantly higher in patients with positive PS than negative PS (327 pg/ml vs. 208pg/ml and 10.5mg/dl vs. 9.8mg/dl respectively, p<0.001), whereas there was no difference in serum phosphorus concentration between groups (p=0.36). Based on ROC curve analysis, the optimal cut-off value to predict scan positivity was 121 pg/ml of serum PTH and 10.3mg/d of Cal. ROC curve analysis showed that diagnostic sensitivity was 74% and 65%; diagnostic specificity was 51% and 66% at this level. PS identified 101 of the parathyroid lesions, whereas, USG detected 71, yielding a sensitivity of 87% and 61% respectively (p=0.008). When PS and USG findings were combined with optimal cut-off values of biochemical parameters the sensitivity improved to 91% and 86% respectively.

Conclusions MIBI scan is more sensitive than USG for the detection of parathyroid lesions in patients with pHPT. By using serum iPTH and Ca values, a higher sensitivity can be achieved.