In nuclear medicine, which radionuclide is used for sentinel lymph node mapping in breast cancer, and what is its mechanism of localization?

• A I-131 — trapped by iodine uptake in lymphoid tissue
• B Tc-99m labeled nanocolloid — uptake by phagocytic cells in sentinel lymph nodes ✓
• C Ga-67 — receptor-mediated uptake in activated lymphocytes
• D Tl-201 — potassium analog uptake by metabolically active nodes

Explanation

Tc-99m labeled nanocolloid (or filtered sulfur colloid) is injected perilesionally in breast cancer for sentinel lymph node mapping. After injection, nanocolloid particles (20-80 nm) are taken up by phagocytic reticuloendothelial cells (macrophages) in the first draining (sentinel) lymph node. The surgeon uses a gamma probe intraoperatively to identify the sentinel node for excision and histological examination, avoiding full axillary clearance in node-negative patients. I-131 is for thyroid pathology. Ga-67 localizes in inflammatory/tumoral tissue by transferrin receptor mechanism.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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