Isotope imaging for metastatic thyroid cancer

Area of Science:

• Nuclear Medicine
• Oncology
• Radiopharmaceuticals

Background:

• Various isotopes are available for imaging thyroid cancer recurrence and metastases.
• TSH-stimulated low-dose iodine-131 (131I) whole-body scanning is the standard for monitoring patients without palpable disease or elevated thyroglobulin on LT4 therapy.

Purpose of the Study:

• To review available isotopes for imaging thyroid cancer recurrence and metastases.
• To discuss the advantages and limitations of different imaging agents.
• To guide the selection of appropriate imaging procedures based on patient presentation.

Main Methods:

• Review of current literature on radioisotope imaging for differentiated thyroid carcinoma.
• Comparison of sensitivity, imaging characteristics, and cost-effectiveness of various agents including 131I, 123I, FDG-PET, 99mTc MIBI, 201Tl, and 99mTc tetrofosmin.
• Discussion of complementary imaging modalities such as ultrasound and CT.

Main Results:

• Low-dose 131I scans are less sensitive than other agents but can identify disease amenable to 131I therapy.
• Iodine-123 (123I) has better imaging characteristics and may replace 131I.
• Fluorine-18-deoxyglucose Positron Emission Tomography (18FDG-PET) shows high sensitivity for cases with elevated thyroglobulin and negative 131I scans, but cost and availability are issues.
• Ultrasound and CT are useful for detecting surgically remediable disease and pulmonary metastases, respectively.
• 99mTc tetrofosmin may offer advantages over 99mTc MIBI due to better image quality and higher sensitivity.

Conclusions:

• TSH-stimulated low-dose 131I scanning remains a preferred initial approach, with 123I as a potential alternative.
• For patients with elevated thyroglobulin and negative 131I scans, 18FDG-PET is the most sensitive option, followed by ultrasound and CT.
• The choice of imaging agent should consider sensitivity, cost, availability, and the potential for radioiodine therapy.