Introduction: Optimal parathyroid scintigraphy requires an understanding of the embryologic, anatomic, and physiologic features of the parathyroid glands and the properties of technetium-99m sestamibi. Normal parathyroid glands are too small to be visualized, but parathyroid disease often produces visibly enlarged glands. Enlarged parathyroid glands may be found near the thyroid gland or outside their expected locations.
The dual-phase protocol consists of performing images about 15 minutes after intravenous 99mTc-sestamibi administration and about 2 hours later.This technique is easy to perform, interpret and economical, and has been shown to provide high sensitivity and specificity , especially in patients with hyperparathyroidism. However, some autors like Carty an colleagues, recently found that early images were more sensitive than delayed ones (92% and 74% respectively). In our institution we traditionally perform an additional 6 hour imaging but never evaluated those images systematically. The primary endpoint of this study is to compare the number of focal areas of uptake identified on each of the tree images (15 min., 2 hours and 6 hours post-injection) performed in each patient. Secondary endpoint was the interobserver agreement.
Methods: We retrospectively evaluated ten patients with primary or secondary hyperparathyroidism who underwent scanning 15 minutes, 2 hours and 6 hours after intravenous administration of 740MBeq of Tc-99m-sestamibi. All images were acquired with a high resolution collimator, 20% window centered on 140KeV, 128x128 matrix, 1000 Kcounts per image. Images were randomly and individually analyzed by two nuclear medicine physicians blinded for timing of imaging and patient identification. They reported the number and location of focal areas of abnormal sestamibi accumulation. Interobserver agreement and the number of abnormal areas in each image were analized and a P < or = 0,05 was considered statistically significant.
Results: Ten patients were evaluated. Fifty percent (5 patients) had positive results on scintigraphy. The 15 minute images were positive in all of these patients. The two hour image was positive in two cases (40% of abnormal studies) and the six hour image in one patient. Phisicians gave different reports in 3.3% (one) of the thirty images.
Discussion: We had no data concerning surgical or laborathory results of our patients. Therefore we couldn't calculate sensitivity or specificity of our results, but they suggest a higher sensitivity of the 15 minute images. In six hours there is a significant washout from the thyroid and probably from enlarged parathyroid glands since only one area of uptake remained positive at this time. This patient had positive 15 minute and two hour images.
Care must be taken not to interpret these phisiological late washout as a negative study. Positive results should be primarily based on the retention of the tracer in the parathyroids compared to the normal thyroid parenquima in 15 minutes and 2 hours after tracer injection.
