18 F-Fluorocholine-Positron Emission Tomography/Computerised Tomography is Useful in Localising 99m Tc-Sesta-methoxyisobutylisonitrile-Negative Parathyroid Cyst Causing Normocalcemic Primary Hyperparathyroidism.

Parathyroid cysts are extremely rare and are rarely associated with primary hyperparathyroidism (PHPT), which are difficult to localise, as they are 99m Tc-sesta-methoxyisobutylisonitrile (sestaMIBI) negative. We report for the first time the utility of 18 F-fluorocholinepositron emission tomography/computerised tomography (PC-PET/CT) in localising parathyroid cyst causing normocalcemic PHPT. A 76-year-old lady with progressively worsening osteoporosis from 2014-2017 (in spite of annual zolendronic acid infusions, daily calcium and vitamin-D supplementation) with persistently normal serum calcium and vitamin D, but elevated parathyroid hormone, had normal sestaMIBI scans of the neck on multiple occasions. FC-PET/CT finally revealed soft tissue uptake, suggestive of right superior parathyroid adenoma/ hyperplasia. Surgical removal of the culprit lesion resulted in resolution of hyperparathyroidism, histopathologic evaluation of which revealed a cystic lesion lined by chief cell variant parathyroid cells without any nuclear atypia, capsular or vascular invasion. FC-PET/CT is useful in localising culprit parathyroid lesions, especially when they are sestaMIBI negative. PC-PET/CT is useful in localising parathyroid hyperplasia and ectopic parathyroids, which are frequently missed by sestaMIBI. There is an urgent need for comparative studies between sestaMIBI and FC-PET/CT in PHPT. We report for the first time the usefulness of FC-PET/CT in localising sestaMIBI-negative functional parathyroid cyst causing normocalcemic PHPT.