Radioisotope mapping is an essential technical component of sentinel lymph node (SLN) biopsy, and most authors define success by an arbitrary threshold SLN-background ratio.
Few studies have examined the degree to which the relative level of SLN counts correlates with the presence of metastasis. Having removed the SLN with the highest counts, there are no data suggesting how far the surgeon should persist in removing additional SLNs that contain much lower levels of isotope.
We performed 134 SLN biopsy procedures in 132 patients with melanoma. Successful isotope localization was defined using an SLN/"hottest" SLN ratio; we defined an SLN as any node containing counts at least 10% of that of the hottest SLN.
Of 83 patients with more than 1 SLN site identified, 21 (25%) had SLNs that contained metastasis. In 17 (81%) of these cases, the SLN with the highest countcontained tumor, but in 4 (19%) it was benign. Among these 4 patients, the counts of the hottest benign SLNs exceeded those of SLNs positive for metastasis on histological examination by a ratio of at least 10:1, and the counts of the positive SLNs were less than 4:1 of those of the background counts or the presence of blue dye failed to identify the positive SLN. No optimum ratio of SLN/SLN or SLN/background counts identified the positive SLN in all cases.
Although the SLN with the highest counts contained metastasis in 81% of patients with malignant melanoma and multiple SLNs, neither a relatively high isotope count nor the presence of blue dye consistently predicted SLN positivity. For maximum accuracy, SLN biopsy requires the removal of all nodes containing isotope regardless of the relative magnitude of counts and the concurrent use of blue dye to salvage those procedures in which isotope mapping fails.