Appearance of intrathymic parathyroid adenomas on pinhole sestamibi parathyroid imaging

Ectopic inferior parathyroid adenomas (PAs) are frequently located in the anterior mediastinum, quite often in the thymus gland. Correct preoperative localization is particularly helpful for surgical planning. Clinical follow up has shown that most intrathymic adenomas were visualized on pinhole views and located closer to the thyroid than expected. We reviewed the typical appearance of intrathymic PA on pinhole views.

We retrospectively reviewed sestamibi pinhole parathyroid imaging performed on 163 patients with primary hyperparathyroidism and final diagnoses established by surgery, histology, intraoperative PTH monitoring, and clinical follow up. Studies showing focal increased activity in the lower pole region of the thyroid were selected and divided into 2 groups, group A (foci that are visually not separable from the thyroid) and group B (foci that are completely separated from the thyroid), and correlated with the final diagnoses.

Of the 163 patients, 102 had 103 clearly abnormal foci in the lower pole region (bilateral lower pole foci in one study). There were 93 foci in group A and 10 foci in group B. Of the 93 foci in group A, there were 80 normally situated inferior PA, 6 descended superior PA, 3 intrathyroidal PA, one hyperplastic parathyroid gland, one thyroid adenoma, one unidentified, and one intrathymic PA. Of the 10 foci in group B, 3 were eutopic inferior PA and 7 were intrathymic PA.

Focal increased activity completely separated from the lower pole of thyroid (regardless of the distance of separation) on sestamibi pinhole images indicates a high probability of intrathymic parathyroid adenoma.