The definition of the sentinel lymph node in melanoma based on radioactive counts

Background There is no consensus on the definition of a hot, nonblue sentinel lymph node (SLN), despite the widespread use of radiocolloid in SLN mapping. Methods A retrospective review of 592 patients with malignant melanoma who underwent SLN mapping was performed. Ex vivo SLN counts and nodal bed counts were obtained by using a gamma probe. The size of each metastatic deposit in an SLN was defined as macrometastases (>2 mm), micrometastases (≤2 mm), a cluster of cells, or isolated melanoma cells. Results A total of 1175 SLNs (SLN⁻, n=1041; SLN⁺, n=134) were evaluated. The mean SLN count/bed counts were SLN⁻, 322±980 and SLN⁺, 450±910 (not significant [NS]) (>2 mm, 270±792 [NS]; ≤2 mm, 446±693 [NS]; isolated melanoma cells/cluster of cells, 677±1189 [P=.036]). Overall, 16 (1.4%) of the SLNs collected had an overall ratio of ≤2. This included two positive SLNs (1.5%), both of which contained macrometatic disease. Forty-seven positive nodal basins had at least one negative SLN. The hottest SLNs in these basins were negative for metastatic disease in nine cases (19.1%). In one basin (2.1%), the positive SLN count was <10% of the hottest lymph node count. Conclusions Removal of lymph nodes until the bed count is 10% of the hottest lymph node will remove 98% of positive SLNs. Lymph node tumor burden influences radioactive counts.