Causes of False-Positive and False-Negative Sestamibi Scans for Parathyroid Imaging
False-negative sestamibi scans occur primarily with multigland disease, small adenomas (<1.3 cm), low serum PTH levels (≤252 pg/ml), mild hypercalcemia, parathyroid hyperplasia, and adenomas with high clear cell or adipose content; false-positive results are most commonly caused by thyroid nodules, thyroid carcinoma, lymphoma, and retained tracer in adjacent tissues.
False-Negative Results: Key Causes
Patient and Disease Factors
The sensitivity of sestamibi imaging is significantly reduced by several clinical scenarios 1:
- Multigland disease (MGD): Sensitivity decreases from 71% to 64% with dual-tracer techniques 1, and sestamibi is notably less sensitive for detecting hyperplastic parathyroid glands compared to solitary adenomas 1
- Mild biochemical disease: Lower serum calcium levels and PTH ≤252 pg/ml correlate with negative or inconclusive results 1, 2
- Concomitant thyroid disease: Nodular thyroid disease significantly impairs detection 1
Lesion Characteristics
Size matters critically: Lesions ≤1.3 cm show markedly reduced sensitivity 2. More granularly, lesion weight directly correlates with detection rates 3:
- Lesions <100 mg: 70.8% false-negative rate
- Lesions 101-300 mg: 51.8% false-negative rate
- Lesions 301-1,000 mg: 34.6% false-negative rate
- Lesions >1,000 mg: Only 8.33% false-negative rate
Cellular and Molecular Factors
The histologic composition profoundly affects tracer uptake 4, 5:
- High clear cell content: Strongly associated with false-negative scans (P=0.01) 5
- High adipose tissue content: Trending association with false-negatives (P=0.1) 5
- Low oxyphil cell content: Oxyphil cells are mitochondria-rich and account for MIBI uptake; fewer oxyphil cells result in lower uptake and rapid washout 4
- P-glycoprotein expression: Parathyroid tissue expressing P-glycoprotein or multidrug resistance-related protein (MRP) does not accumulate MIBI effectively 4
Technical Factors
Sensitivity ranges from 24% to 66% when uptake is masked by retained radionuclide in adjacent thyroid or submandibular gland tissue 1.
False-Positive Results: Key Causes
Thyroid Pathology (Most Common)
Solid thyroid nodules are the most frequent cause of false-positive MIBI results 1, 4. Specific thyroid conditions include:
- Follicular thyroid adenomas: Demonstrate increased MIBI uptake 6
- Papillary thyroid carcinoma (including follicular variant): Can mimic parathyroid adenomas 7
- Thyroid carcinoma in general 4
Other Causes
- Lymphoma 4
- Lymphadenopathy 4
- Retained isotope in veins: Can create the illusion of ectopic adenoma; injecting tracer in the contralateral arm can overcome this artifact 8
- Hibernoma (rare): One case reported 3
Clinical Implications and Mitigation Strategies
Improving Accuracy
Adding SPECT/CT to planar imaging significantly reduces false-positives from thyroid disease 1. The sensitivity and PPV of sestamibi/I-123 subtraction techniques are least negatively impacted by concomitant thyroid disease when SPECT/CT is included 1.
Dual-tracer subtraction techniques help differentiate thyroid from parathyroid lesions: Many MIBI-avid thyroid lesions also accumulate pertechnetate or I-123, allowing subtraction to reduce false-positives 1, 4.
Important Caveats
- Not all parathyroid lesions retain MIBI, and not all thyroid tissue washes out quickly 4
- In secondary hyperparathyroidism, MIBI uptake relates more closely to cell cycle than gland size 4
- Intrathyroidal or ambiguous localization should raise suspicion for thyroid pathology rather than parathyroid disease 7
- In endemic goiter areas, expect higher rates of false-positives due to increased prevalence of thyroid adenomas 6
When Sestamibi Fails
In cases of recurrent disease where sestamibi cannot localize a focus, consider FNA with PTH washout of suspicious lesions identified on ultrasound 9. This is particularly important in parathyroid carcinoma cases where metastases may not demonstrate typical sestamibi avidity.
Optimal Imaging Strategy
Combining neck ultrasound with sestamibi SPECT/CT achieves the highest accuracy (96.2%) for preoperative parathyroid localization 2, compensating for the limitations of each modality individually.