SUN-827 - Not All That Glitters Is Gold: False-Positive Mediastinal Sestamibi Uptake in A High-Risk Thymoma

Primary hyperparathyroidism is most commonly caused by a single parathyroid adenoma, and the definitive treatment is parathyroidectomy. Preoperative localization is typically guided by technetium-99m (Tc-99m) sestamibi scintigraphy, which identifies adenomas with approximately 74% sensitivity and 90% specificity. However, sestamibi uptake can occur in non-parathyroid lesions that have high metabolic activity, such as thymomas and thyroid nodules, creating the potential for misleading localization.

We present a case of a 69-year-old man initially evaluated for right thyroid nodules that were consistent with an oncocytic follicular neoplasm on fine-needle aspiration, for which a right hemithyroidectomy was planned. Preoperative laboratory testing revealed concurrent primary hyperparathyroidism with a serum calcium of 11.4 mg/dL, parathyroid hormone (PTH) of 136 pg/mL, and 24-hour urine calcium of 173 mg. Tc-99m sestamibi imaging revealed uptake in both the right thyroid nodule as well as in a 1.4 cm anterior mediastinal mass, raising concern for an ectopic parathyroid adenoma in the thymus. Given these findings, mediastinal exploration was planned and performed immediately prior to the hemithyroidectomy through collaboration between endocrine and thoracic surgery teams. Mediastinal exploration removed what was later determined to be a type B3 stage IIa thymoma without parathyroid tissue on final pathology. Intraoperative PTH remained elevated following the thymectomy, prompting further exploration. Subsequent cervical exploration identified a right inferior parathyroid adenoma, with PTH decreasing from 117 to 23 pg/mL after excision. Right hemithyroidectomy was then performed as planned, with final pathology consistent with microscopic papillary thyroid carcinoma.

This case highlights the importance of operative sequencing when sestamibi findings are atypical. Performing mediastinal exploration first revealed a thymoma rather than an ectopic parathyroid adenoma. Had the cervical procedure been done first, a second neck operation would have been required after discovering persistent PTH elevation post-thymectomy. Beginning with the mediastinal operation established the diagnosis and enabled definitive treatment without need for re-operation. Intraoperative PTH monitoring and multidisciplinary coordination were also critical for true localization and management of complex primary hyperparathyroidism.

When sestamibi findings demonstrate mediastinal uptake, multidisciplinary coordination and deliberate operative sequencing guided by intraoperative PTH monitoring are critical to avoid an unnecessary second neck exploration and ensure definitive management.

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