I-131 vs I-123 for Post-Thyroidectomy Whole-Body Scans
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Question
After thyroid cancer surgery (total thyroidectomy) and radioiodine ablation with I-131, a diagnostic whole body scan is performed to detect residual or metastatic thyroid tissue. In this context, I-131 is preferred over I-123 because:
- AI-131 has better image quality due to higher photon energy
- BI-131 does not require thyroid hormone withdrawal
- CI-131's longer half-life (8 days) allows delayed imaging at 5-7 days to detect metastases
- DI-131 is not taken up by normal tissue, reducing background activity
Show the answer and explanation
C. I-131's longer half-life (8 days) allows delayed imaging at 5-7 days to detect metastases
For post-therapy or diagnostic whole body scanning in differentiated thyroid cancer, I-131 is commonly used because its 8-day half-life permits delayed imaging at 3–7 days after administration. This delay allows normal background activity to clear, improving detection of residual thyroid tissue and distant metastases. I-123 (13.2-hour half-life) can only be imaged at 24 hours, limiting delayed scan capability. Thyroid hormone withdrawal or recombinant TSH stimulation (Thyrogen) is required with EITHER isotope to elevate TSH and stimulate iodine uptake.
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