Whole-Body Iodine Scan for Differentiated Thyroid Cancer (DTC)

A whole-body iodine scan (WBS) is a nuclear medicine imaging test used to detect residual thyroid tissue, metastases, or recurrence in patients with differentiated thyroid cancer (DTC) (e.g., papillary or follicular thyroid cancer). The scan uses radioactive iodine (I-131 or I-123), which is selectively taken up by thyroid cells and thyroid cancer cells.

Indications for Whole-Body Iodine Scan:

✔ Post-thyroidectomy surveillance (detecting residual thyroid tissue).
✔ Detection of metastases (lungs, bones, lymph nodes, etc.).
✔ Monitoring recurrence after radioiodine therapy.
✔ Planning for radioactive iodine (RAI) therapy (determining iodine-avid lesions).

Procedure:

1. Thyroid-Stimulating Hormone (TSH) Elevation:

   • Endogenous stimulation: Withdrawal of thyroid hormone (levothyroxine) for 4-6 weeks.

   • Exogenous stimulation: Administration of recombinant human TSH (rhTSH, Thyrogen) while continuing thyroid hormone therapy.

2. Administration of Radioactive Iodine:

   • I-131 (for diagnostic or post-therapy scans) – Higher radiation dose, longer half-life.

   • I-123 (for diagnostic scans) – Lower radiation dose, better image quality.

3. Waiting Period:

   • 24-72 hours after iodine administration, allowing uptake in thyroid tissue or cancerous lesions.

4. Imaging with Gamma Camera (Planar/SPECT-CT):

   • Whole-body images are taken to detect iodine-avid areas.

Interpretation of Results:

• Normal: Only residual uptake in the thyroid bed (post-thyroidectomy patients).

• Positive for Metastasis:

  • Neck or mediastinal uptake: Suggests lymph node metastases.

  • Lung uptake: Suggests pulmonary metastases.

  • Bone uptake: Suggests skeletal metastases.

• Negative for Uptake:

  • No iodine-avid disease → Consider FDG-PET scan (for iodine-refractory thyroid cancer).

Advantages of Whole-Body Iodine Scan:

✅ Highly specific for iodine-avid thyroid cancer.
✅ Helps guide radioactive iodine therapy for metastatic disease.
✅ Detects distant metastases early.