Misleading localization by 18F-fluorocholine PET/CT in familial hypocalciuric hypercalcemia type-3: a case report

Abstract

Background: Familial hypocalciuric hypercalcemia (FHH) is a heterogeneous autosomal-dominant disorder of calcium hemostasis that may be difficult to distinguish clinically from mild primary hyperparathyroidism. Loss-of-function mutations mainly involving Arg15 residue of the adaptor-related protein complex 2, sigma subunit 1 (AP2S1) cause a rarer, more recently recognized form of FHH, FFH type-3. Recently, ¹⁸F-fluorocholine positron emission tomography/computed tomography (FCH-PET/CT) showed superior sensitivity to conventional imaging in localizing parathyroid adenomas. We report a new FFH type-3 patient who underwent unnecessary parathyroidectomy in association with misleading FCH-PET/CT imaging.

Case presentation: A 29-year old woman was initially evaluated for parathyroid hormone (PTH)-dependent hypercalcemia in 2013. Medical history was positive only for chronic constipation and malaise with no personal or family history of hypercalcemia, kidney stones, or neck surgery. Over seven years, serum calcium level was 2.51-2.89 mmol/L with concomitant PTH level of 58.7-94.8 mmol/L. Serum phosphate levels were in the low/low normal range. Serum creatinine and magnesium levels were normal. 25-hydroxy vitamin D level was 13 nmol/L. 24-hour urine calcium level was 1.92 mmol/day but increased to 6.99 mmol/day after treatment with cholecalciferol 1000 IU daily. Bone mineral density and renal ultrasound were normal. Parathyroid ultrasound showed two hypoechoic nodules inferior to the left and right thyroid lobes; however, ^99mtechnitium-sestamibi scans (2013, 2016, 2018) were negative. FCH-PET/CT (2019) showed focal uptake co-localizing with the nodule inferior to the left thyroid lobe. The patient underwent left inferior parathyroidectomy and pathology was consistent with parathyroid hyperplasia. However, postoperatively, serum calcium and PTH levels remained elevated and FCH-PET/CT and ultrasound showed persistence of the uptake/nodule. Whole exome sequencing showed Arg15Cys mutation in the AP2S1 gene characteristic of FHH type-3.

Conclusions: In this new case of FHH type-3, FCH-PET/CT failed to localize to the hyperplastic parathyroid glands and localized instead to apparently a lymph node. This, together with increased urinary calcium after vitamin D supplementation, led to unnecessary parathyroidectomy. Given the increasingly lower cost of genetic testing and the cost of follow up and unnecessary surgery, it may prudent to include genetic testing for FHH early on in patients with mild PTH-dependent hypercalcemia.

Keywords: 18F-fluorocholine positron emission tomography/ computed tomography; False positive localization; Familial hypocalciuric hypercalcemia type-3; Primary hyperparathyroidism.

Fig. 1

Parathyroid US. Sagittal view (a and b) showing an approximately 1.0 cm vascular hypoechoic nodule inferior to the left thyroid lobe

Fig. 2

^99mTechnetium-sestamibi dual phase scan. a, anterior view twenty minutes after tracer administration showing homogeneous tracer distribution within the thyroid gland (red arrow) and normal variant brown fat uptake (blue arrows). b, 2-h delayed anterior view showing complete wash out of the tracer from both thyroid lobes, persistent brown fat uptake (blue arrows), and no abnormal tracer activity in the thyroid bed or elsewhere in the head, neck, and the mediastinum to qualify for parathyroid adenoma

Fig. 3

¹⁸F-fluoroholine PET/CT. Images were obtained before (a, b, and c) and after (d, e, and f) left inferior parathyroidectomy. Coronal PET (a and d), axial CT (b and e), and fused PET/CT (c and f) images show choline-avid lesion inferior to the lower pole of the left thyroid lobe (blue arrows) that persisted after parathyroidectomy