Why use Single Photon Emission Computed Tomography (SPECT) Computed Tomography (CT) in addition to whole body Iodine-131 (I-131) scans?

SPECT/CT in Addition to Whole Body I-131 Imaging for Thyroid Cancer

SPECT/CT should be used in addition to whole body I-131 scans because it significantly improves diagnostic accuracy, provides precise anatomical localization of radioiodine uptake, and changes patient management in up to 35.6% of cases. 1

Benefits of Adding SPECT/CT to Whole Body I-131 Imaging

• SPECT/CT provides better identification and interpretation of radioiodine uptake foci, more correct anatomic localization, and precise differentiation between tumor lesions and physiologic uptake 1

• SPECT/CT can detect additional occult foci not visible on planar whole-body imaging, with studies showing detection of 28% more lesions compared to planar imaging alone 1

• SPECT/CT enables accurate characterization of equivocal findings on planar imaging, correctly identifying whether uptake represents residual thyroid tissue, lymph node metastases, or physiologic uptake 2

• The addition of CT allows for attenuation correction and facilitates precise localization of any focus of increased tracer uptake, particularly useful for abdominal imaging 3

Clinical Impact on Patient Management

• SPECT/CT has been shown to modify therapeutic management in 35.6% of positive cases and avoid unnecessary treatment in 20.3% of patients with benign lesions or physiologic uptake 1

• SPECT/CT can reduce the need for additional cross-sectional imaging studies in approximately 20% of patients, leading to cost savings and reduced radiation exposure 4

• In patients with high serum thyroglobulin levels, SPECT/CT provides valuable information for detecting hidden metastases even when planar imaging suggests only thyroid remnant tissue 5

• SPECT/CT can change the American Thyroid Association risk of recurrence classification in 6.4% of post-surgical patients, altering follow-up recommendations 4

Specific Clinical Scenarios Where SPECT/CT Adds Value

• For suspected recurrence of differentiated thyroid cancer, the American College of Radiology recommends both ultrasound and I-123 whole body scans as complementary procedures, with CT neck with IV contrast or MRI neck as appropriate second-line studies 3

• SPECT/CT is particularly valuable for:

  • Differentiating between residual thyroid tissue and lymph node metastases in the neck 2
  • Distinguishing between lung versus mediastinal metastases 2
  • Precisely localizing skeletal metastases 2
  • Characterizing equivocal foci of uptake in the abdomen 3

• SPECT/CT can detect thyroid cancer lesions in patients with serum thyroglobulin levels below the typical treatment threshold (1 μg/L), identifying patients who would otherwise not receive treatment 6

Technical Considerations

• If only one SPECT scan is performed, acquisition at 24 hours post-injection is preferred because of a higher target-to-background ratio 3

• SPECT/CT images are obtained for a 360° orbit with appropriate matrix and angle steps (typically 128×128 word matrix, 6° angle steps, 30–45 seconds per stop) 3

• Coregistered CT images (100–130 kV, mAs modulation recommended) enable attenuation correction and precise localization 3

Pitfalls and Limitations

• False positives can occur due to physiologic uptake in salivary glands, thymus, liver, and gastrointestinal tract 3

• Laxatives are advised when imaging the abdomen to reduce difficulties in interpretation due to bowel activity 3

• The CT portion of SPECT/CT may demonstrate non-iodine-avid lesions in approximately 22% of patients, providing additional diagnostic information 4

• While SPECT/CT provides significant benefits, it does increase radiation exposure compared to planar imaging alone, so patients should be well hydrated before and for at least 1 day after injection to reduce radiation exposure 3