'Shine through' on dual tracer parathyroid scintigraphy: a potential pitfall in interpretation.

Dual tracer scintigraphy has become the procedure of choice for preoperative localization of parathyroid adenomas. The typical protocol used by most laboratories is to initially perform a Tc-99m sestamibi (MIBI) study with early and delayed (2-3 hours) washout phase images. This is followed by a thyroid-specific Tc-99m pertechnetate study. A potential pitfall exists if the delayed phase of the initial MIBI study shows an intense, abnormal focus of retention. This focus may interfere with interpretation of the subsequent pertechnetate study because of a "shine through" effect. To differentiate a parathyroid adenoma from a thyroid adenoma, it may be necessary to repeat the thyroid-specific pertechnetate scan on a separate day after the dual tracer study.