Answer
Answer
A false-positive result may be caused by contamination of the skin or residual activity from another nuclear medicine procedure done recently.
A false-negative result (absent or faint lymph node uptake) may be caused by a low radiopharmaceutical dose, poor radiopharmaceutical quality, insufficient radiocolloid particles, imaging time that is too early or too late, advanced patient age, or tumor replacement of nodes.
One way of reducing false-positive and false-negative results is to combine the visual assessment of nodes with intraoperative gamma counting and digital palpation through the surgical incision. [15]
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Media Gallery
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Injecting syringe.
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Gamma probe with a highly collimated handheld detecting wand, which is covered with a sterile cover for use within the surgical field.
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Injection tray containing the radioactive dose in a syringe shield, alcohol swabs, bandage, and dry cotton.
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Intradermal injection technique. Note that the needle tip is within the skin thickness and a bleb has formed on the skin. The cancer is represented by the black mass.
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Intradermal injection technique. The needle is inserted at a very acute angle with the skin and the bleb forms when the radiocolloid is injected.
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Intraparenchmal injections. Note the needle injects within the parenchyma around the lesion.
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Postinjection massage. Dry cotton is used to massage the injection site to encourage lymph flow with the radiocolloid towards the sentinel node.
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Methylene blue is injected in the operating theater. Note the larger volume and a deeper insertion angle.
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The sentinel node is located using the gamma counter probe in the area with the highest count rate. This is done after sterilization; note that the probe is covered with a sterile sheath.
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With the probe in the proper location, the area is marked on the skin.
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The incision is made over the area identified by the mark on the skin.
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The incision leads to a blue lymph node. Note the blue channel at 1 o'-clock position indicating that this is the sentinel node (to be corroborated by radiation counting).
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The blue node is confirmed to be the sentinel node by counting over it. It should be the "hottest" node.
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The blue and hottest node is excised and ex vivo counts are checked. Note that the probe is directed towards the ceiling to avoid getting extraneous counts from the patient. Note the high counts (acquired over 2 seconds) at the top of the screen.
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After the sentinel node is excised, the nodal bed is checked again for residual counts. If the sentinel node has been removed, the count rate should fall to the background level or 10% of the in vivo count rate.
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Postexcision counts are acquired. In this image, the counts have fallen to very low background rates.
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