When is a technetium-99m (Tc‑99m) sestamibi scan indicated for parathyroid evaluation?

Tc-99m Sestamibi Scan for Parathyroid: Primary Indication

Tc-99m sestamibi scanning is indicated for preoperative localization of hyperfunctioning parathyroid glands in patients with confirmed primary hyperparathyroidism (PHPT) who are planned for surgery. 1

When to Order the Scan

The scan should be performed only after:

• Biochemical diagnosis of PHPT is confirmed (elevated PTH with hypercalcemia)
• A surgical indication exists based on meeting at least one criterion:
  • Bone disease: T-score ≤-2.5 SD or low-energy fracture
  • Kidney disease: GFR <60 mL/min/1.73m², nephrolithiasis, or nephrocalcinosis
  • Hypercalciuria: >250 mg/day (women) or >300 mg/day (men)
  • Severe hypercalcemia: calcium >0.25 mmol/L above upper normal limit 1

Critical caveat: If surgery is ruled out, imaging is not indicated 1. The scan serves no purpose in patients managed conservatively.

Recommended Imaging Protocol

First-line approach combines parathyroid ultrasound with functional imaging 1:

Choice of Nuclear Medicine Study:

1. Dual-phase Tc-99m sestamibi with SPECT/CT (preferred technique)

   • Sensitivity: 86% with SPECT/CT vs. 70% planar alone 2
   • Ideally performed as dual-isotope subtraction (123I/99mTc-sestamibi) 1

2. Alternative: 18F-choline PET/CT

   • Equivalent or superior performance to sestamibi 1
   • Preferred first-line in specific situations:
     • Age <40 years
     • Family history or genetic mutations (MEN1, CDC73)
     • Suspected multiglandular disease
     • No access to dual-isotope subtraction scintigraphy 1

Performance Characteristics

Sensitivity varies by clinical scenario:

• Single adenoma: 79-96% 2
• Multiglandular disease: 45-58% (significantly reduced) 2, 3
• Reoperative cases: 92% 3

Factors that decrease sensitivity:

• Small adenomas (<300 mg) 2, 4
• Concomitant thyroid nodular disease 2
• Mild hypercalcemia 2
• Multiglandular hyperplasia 2, 3

Factors that increase detection:

• Higher PTH levels (>147.7 pg/mL: 83% sensitivity) 5
• Elevated calcium with decreased phosphorus 5
• Severe bone disease (100% positive rate, often shows early uptake) 4

Algorithmic Approach When Initial Imaging is Negative

If both ultrasound and sestamibi are negative:

1. Second-line: 18F-choline PET/CT (if sestamibi was first) 1
2. Third-line: 4D-CT or MRI at expert center 1
3. Consider surgical exploration by experienced surgeon if imaging remains negative but surgical indication persists

Important Clinical Pitfalls

Do not use sestamibi for:

• Diagnosis of PHPT (biochemical diagnosis comes first)
• Pediatric patients with familial PHPT or neonates (minimal utility) 2
• Screening purposes without confirmed biochemical disease

Technical considerations:

• SPECT/CT acquisition significantly improves localization over planar imaging alone 2
• Positive predictive value remains high (86-96%) even when sensitivity varies 3, 5
• The scan localizes to correct quadrant or ectopic location, guiding surgical approach 2

The bottom line: Sestamibi scanning is a preoperative mapping tool, not a diagnostic test. Order it only when surgery is planned to guide the surgical approach and enable focused parathyroidectomy rather than bilateral neck exploration.