Lymphoscintigraphy is more effective using higher specific activity (99m)Tc-antimony trisulfide colloid in the rat

Technetium-99m-antimony trisulfide colloid ((99m)Tc-ATC) was investigated with varying specific activity in the rat lymphatic system, as well as its binding to rat blood cells in vitro. Low, moderate and high doses of (99m)Tc-ATC were subdermally injected into tails of rats (n=3) and 30min later whole body lymphoscintigraphic images were acquired. (99m)Tc-ATC was incubated for 30min at 37°C with whole blood (n=4) as for control, at 4°C as for hypothermia, as opsonized at 37°C and lipopolysaccharide (LPS) at 37°C. Cell types were separated by gradient centrifugation to assay for cell bound-radioactivity. These experiments were repeated using mixed leukocytes that were erythrocytes-free. Images showed direct drainage of (99m)Tc-ATC from the tail to the popliteal nodes, sacral node and beyond, along the para-aortic chain, and to a left inguinal lymph node that drained into the axillia. The higher specific activity dose resulted in more visible lymph nodes after 30min. (99m)Tc-ATC was predominantly not cell-bound. In rat whole blood (99m)Tc-ATC-neutrophils were only present to the extent of 3% for the control, 3% for the hypothermia test, 4% for the opsonized and 7% for the LPS test. In the erythrocytes-free samples, (99m)Tc-ATC-leukocytes were present to the extent of 5% (control), 4% (hypothermia), 6% (opsonized) and 11% (LPS). In conclusion, a higher specific activity radiocolloid may identify the sentinel lymph node sooner during lymphoscintigraphy, and there may be an advantage in detecting more counts with the γ-probe. Retention of (99m)Tc-ATC in lymph nodes in vivo is likely because of binding on macrophage surfaces, rather than from internalization by phagocytosis within a time frame of 30min after injection.