The physical characteristics of the probe must be such that detection of radioactive lymph nodes is optimized. The following aspects are important:
• The uptake of radioactivity in lymph nodes is highly variable. Uptake in the sentinel node in melanoma patients varies between about 40 Bq* and 250 kBq with a median of 13 kBq (22 h after injection of 58 MBq 99mTc) .
• The depth of the sentinel node is variable. It may be just below the skin or more than 5 cm deep.
• The activity in the tissue surrounding the lymph nodes is very low, but ''background'' radiation can be considerable if a hot injection site is close to the sentinel node. In particular, this will be the case when the primary tumor in the breast is located in the upper outer quadrant, close to the axilla.
If the injection site is far from the node (or if it can be excised before localization of the sentinel node) and if the sentinel node has a high uptake and/ or is close to the surface, locating the sentinel node will be straightforward. Any probe will locate it. In practice, conditions are usually less favorable. Then, the characteristics of the probe will determine the probability of successful localization. Focusing on these less favorable circumstances, we will discuss the physical characteristics of the probe.
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