Validation of gamma probe detection of the sentinel node in melanoma

In melanoma, the presence or absence of metastasis in the first lymph node (sentinel node, SN) has a predictive value for the entire lymph node basin. This study explores the efficacy of lymphoscintigraphy with 99mTc-nanocolloid and a gamma-ray detection probe in tracing SNs.

Sixty patients with clinically localized melanoma were studied. Lymphoscintigraphy was performed after intradermal injection of 60 MBq 99mTc-nanocolloid at the primary tumor site. Scintigraphy included early dynamic images and a body scan 2 hr postinjection. The following day, a gamma detection probe (Neoprobe 1000) was used intraoperatively to trace the still radioactive SNs. The number of counts of the nodes and the surrounding tissues was measured before, during and after excision. Excised nodes and normal tissue samples were measured in a gamma well counter. The uptake of 99mTc-nanocolloid was calculated.

Lymphoscintigraphy showed 122 SNs distributed over 73 drainage basins. Use of the probe led to retrieval of all nodes that were searched for. The SN-to-background ratios were high: a median of 36 in vivo (range: 2-722) and a median of 274 ex vivo (range: 6-2,985). Counts in vivo correlated well with counts ex vivo. The mean percentage of the injected dose per SN was 0.69 (range: 0.0013-6.82), versus 0.23 (range 0.0004-2.59) in 23 measured second-echelon nodes (non-SNs). Mean percentage of uptake per gram tissue in SNs was 2.1 (range: 0.003-17.4), in skin 0.01 (range: 0.00-0.22) and in subcutaneous fat 0.0035 (range: 0.00-0.081).

Average uptake of 99mTc-nanocolloid in SNs is substantially higher than uptake in non-SNs, skin and subcutaneous fat. The resulting high SN-to-background ratios facilitate the intraoperative detection of these nodes using a gamma detection probe.