Technetium-99m-Sestamibi Parathyroid Scintigraphy: Effect of P-Glycoprotein, Histology and Tumor Size on Detectability

The purpose of this study was to evaluate the effect of P-glycoprotein (P-gp) levels, predominant histology and tumor size on the detectability of parathyroid adenomas with 99mTc-sestamibi scans. Although previous studies have shown that positivity correlates with tumor size, false-negative studies have been reported with large tumors and true-positives reported with very small tumors. Recent investigations have reported rapid washout of sestamibi in malignant tumors because of high levels of P-gp, similar to that seen with multidrug chemotherapy resistance. Therefore, we postulated that this mechanism might account for false-negative studies in parathyroid tumors. Preliminary reports have suggested that the predominant histological subtype influences positivity on 99mTc-sestamibi parathyroid scans. We studied 25 patients with surgically confirmed parathyroid adenomas with 99mTc-sestamibi parathyroid scans. The results of the imaging study were correlated with tissue P-gp levels, predominant histological subtype and size of the surgically removed glands. There were 21 true-positive and 4 false-negative results. The size of the adenomas ranged from 0.12 to 8.64 ml. We found no correlation between the results of the dual-phase 99mTc-sestamibi study and either the predominant cell type or the level of P-gp. Positivity did correlate with the size of the adenoma (p=0.73, p < 0.0001). We cannot exclude the possibility that P-gp and cell type may still play a role in individual cases, but we suspect that other yet to be determined factors may influence 99mTc-sestamibi detectability in addition to tumor size.