Endocrine Abstracts

Introduction: Localization of parathyroid adenomas in primary hyperparathyroidism (PHPT) is achieved commonly using ^99mTc-methoxyisobutylisonitrile with SPECT/CT (MIBI) and neck ultrasound (USS). Drugs interfering with calcium uptake such as calcium channel blockers or cinacalcet may interfere with the sensitivity of MIBI. The aim of our study was to assess the outcomes of MIBI in patients with severe PHPT who are treated with cinacalcet.

Methods: Data on all patients treated with cinacalcet for PHPT for any indication was collected. Patients who had MIBI scan after cinacalcet initiation were included. Data on PTH and calcium and concurrent ultrasound (USS) were collected

Results: n = 35 had MIBI after cinacalcet initiation (total cinacalcet patients=121) Mean duration between cinacalcet initiation and MIBI was 194 days. 63% (n = 22) of MIBI scans were positive despite being on cinacalcet. MIBI was positive in 13/22 patients despite the calcium normalizing. Comparing the MIBI-positive vs. MIBI-negative, baseline PTH was higher (25.1±18.8 vs. 16.0±10.9 pmol/l); baseline adjusted calcium was comparable (3.02±0.17 vs. 3.05±0.25 mmol/l), both not statistically significant. In MIBI-negative patients, USS was also negative, suggesting this may be true negative. In patients with calcium ≥ 3mmol/l, MIBI was positive in 65% (13/20); in patients with calcium ≥ 2.85mmol/L with symptoms, MIBI was positive in 63%.

Conclusion: Cinacalcet is indicated in patients with severe hypercalcemia and contributes to symptom relief and admission avoidance. Pausing treatment to avoid interference to MIBI may be clinically challenging. With 65% of patients generally still getting a conclusive positive result, a pragmatic approach would be to perform MIBI on cinacalcet. Concurrent imaging with USS or alternate imaging modality scans could be performed as initial options to validate a negative MIBI scan; repeating MIBI after pausing cinacalcet treatment could be considered if deemed safe, practicable and necessary.