Once a melanoma has progressed beyond Stage II, it has spread beyond the original site. It is most likely to have reached the lymph nodes that are closest to the tumor.
Palpable nodes. To find out whether melanoma cells have escaped the primary tumor, the physician starts by feeling the nearby lymph nodes. When there is an enlargement or lump in a lymph node, it is described as “palpable,” meaning that the physician can feel it on physical examination.
Today, a lymph node that is palpable is almost always surgically removed. It is then sent to the pathology laboratory to be tested microscopically for the presence of malignant cells. If any are found, the rest of the nodes in that basin will also be removed, and treatments that stimulate the immune system and/or chemotherapy will be recommended.
Non-palpable nodes. The lymph nodes are not always palpable even when melanoma cells have spread beyond the original tumor. In the past, there was much debate about when to excise and examine the local lymph nodes. Some believed in a wait and see policy; others believed in removing all the nodes (radical node dissection) in the region of the tumor on the chance there were hidden cancer cells; this was called elective lymph node dissection, or ELND.
Lymphoscintigraphy (Lymphatic Mapping) and Sentinel Node Biopsy
Today, there are specific guidelines for when to investigate the regional lymph nodes: Two techniques refined over the past decade, called lymphoscintigraphy and sentinel node biopsy, have solved the problem of whether or not to perform radical lymph node dissection in the absence of clinically palpable nodes.
Generally, when patients have melanomas under 1 mm in thickness, with no ulceration and a mitotic rate of less than 1/mm2, nodal dissection is deemed unnecessary. For melanomas that have reached 1 mm in thickness, and/or have ulceration or a mitotic rate of 1/mm2 or greater, lymphoscintigraphy and sentinel node biopsy (SLNB) are undertaken to determine whether all the nodes in the regional lymph node basin should be removed.
Lymphoscintigraphy is a technique for mapping the lymphatic pathway to track whether melanoma cells have metastasized from the primary melanoma tumor to the local lymph nodes. A small amount of a harmless radioactive substance is injected at the site of the melanoma to trace the flow of lymph fluid draining from it to the nodes. Then, with the help of a scanner, the drainage pattern of the lymph fluid is determined.
Most often, a second lymphatic mapping technique is then also used to increase certainty: Blue dye is injected into the skin around the tumor, and the dye passes into the lymph fluid, tracing its path. The blue color is picked up first by the node closest to the tumor, which is referred to as the sentinel node. Sometimes there are one or more other sentinel nodes as well, which should also show up in the dye and radioactive tracer tests. Armed with the findings from this lymphatic mapping, the surgeon can at first remove only the sentinel nodes.
Once a specific area (basin) of lymph drainage has been pinpointed by the dye or tracer, the sentinel node(s) can be removed surgically and tested in the pathology laboratory, the premise being that if any melanoma cells reach the local nodal basin, they will show up in the sentinel node(s). If no cancer cells are found in the sentinel nodes, no further surgery is performed. If cancer cells are present in the sentinel nodes, the rest of the nodes in this lymphatic basin will also usually be removed and examined. Once melanoma cells are confirmed in the lymph nodes, the patient is reclassified as Stage III.
Microscopic nodal involvement
Research is now also exploring special biochemical techniques that can identify those melanoma cells that do not show up in the course of routine microscopic examination and sentinel node biopsy.
Local vs. Distant Spread
Once the disease has advanced to Stage IV, melanoma cells have traveled through the body via the bloodstream or lymph vessels, going far from the original tumor site. They may have reached distant lymph nodes or invaded the internal organs. This can be in addition to or instead of in-transit metastases or local spread to the regional lymph nodes. In local forms of the disease, the metastases can reach skin or subcutaneous tissue more than 2 cm from the primary tumor, but not beyond the regional lymph nodes.
When distant metastases are suspected, they can be traced by scans of the chest, head, abdomen, and pelvis with a CT (computed tomography) scan in which special X-ray equipment and a computer program show a cross-section of body tissues or organs; an MRI (magnetic resonance imaging) scan that uses a magnet instead of X rays to create a map of the patient’s body and brain; and by PET (positron emission tomography), an evolving radiographic technique. For PET scanning, radioactive sugar, the basic carbohydrate utilized by the body for energy, is injected intravenously into the patient. This sugar may be taken up rapidly by any melanoma cells that are present.