Once the channels have been defined, their course can be marked on the skin of the patient, if the surgeon plans to excise these channels. In most patients, however, these are not removed surgically and treatment involves wide local excision of the primary excision biopsy site followed by lymph node dissection, if the risk of nodal metastases is high.
In addition to the channels, interval nodes (nodes along the channel but not in the lymph node fields) and sentinel nodes (the nodes to which the lesion directly drains) are also detected and their location marked on the skin. The depth of the sentinel nodes beneath the skin surface is measured on the scan.
This technique allows draining node fields to be accurately sampled for the presence of metastases with the minimum of surgery. It also ensures that all relevant material is removed, even if the path taken through the system or the draining node fields themselves are different from those predicted by traditional methods.
The results of this technique, which was performed on over 1400 patients, were recorded in a spreadsheet and then transferred onto schematic maps of the body using ArcView. The images were used to examine some of the commonly held perceptions about the node fields to which lesions on various parts of the body drain.
Traditional medical concepts of lymph node drainage paths date back to 1843 when Sappey injected cadavers with mercury to trace the paths taken through the lymphatic system from various points on the body (Sappey 1843 cited in Uren, "Lymposcintigraphy in High Risk Melanoma of the Trunk: Predicting Draining Node Groups, Defining Lymphatic Channels and Locating the Sentinel Nodes," Journal of Nuclear Medicine, Vol. 34, 1993). Lymphoscintigraphy has shown these concepts to be incorrect in a large proportion of patients.
Mapping the primary lesions and their draining node fields allows the researcher to quantify and analyze the divergence of paths actually taken from those predicted to diverge. Plots of all primary lesions draining to a particular node field can be used to establish the general pattern of distribution. With the addition of color, it can be shown that the lines traditionally used to delineate watershed boundaries in the lymphatic system are incorrect.
Melanoma depends almost exclusively on surgical treatment. After a biopsy has revealed malignant melanoma, a wide local excision of the area surrounding the primary lesion site is performed. Because the thickness of the melanoma is the most important prognostic factor, the margins for this excision increase with the thickness of the original melanoma.
The thicker the melanoma, the worse the prognosis and the more likely the presence of metastases in the draining node fields. In patients with intermediate thickness melanoma (between 1 mm and 4 mm), about 30 percent will have micrometastases in the draining lymph nodes. Elective lymph-node dissection in this group of patients would thus involve unnecessary surgery in
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