Melanoma is an aggressive type of skin cancer. The tumor arises in pigmented cells in the skin called melanocytes. For reasons that we do not fully understand, these cells become cancerous, although sun exposure is thought to have a causative role. Melanoma usually forms at the site of a pre-existing mole; however, this is not always the case. Any mole that grows rapidly, bleeds, ulcerates, has an irregular margin, or an unusual color distribution should be considered suspect and a biopsy performed. Melanomas can form on any part of the body, but the sun-exposed areas are the most common. They may also form on the palms of the hands, on the soles of the feet, beneath finger and toe nails, and in the mouth, genital, or perianal areas. Melanomas are more common in Caucasians than in African-Americans, with a ratio of 20:1. The best treatment for melanoma is prevention through avoidance of prolonged sun exposure or use of strong sun blocking lotions if prolonged exposure is anticipated.
The extent of the malignancy at the time of diagnosis is the most important prognostic factor. Patients with disease confined only to the skin have the best prognosis while those with disease confined to the skin and adjacent lymph nodes, but no distant spread, have an intermediate prognosis. Patients who have disease spread to distant organs, i.e. liver, or bone, have a poor prognosis. The depth of tumor invasion (the thickness of the original lesion) helps predict the likelihood of spread, as well as the surgical treatment. Tumors less than 1.0 mm. thick have a low risk of spread, those 1.0 – 4.0 mm in thickness (intermediate lesions) have at least a 20% chance of spread to the closest lymph nodes, even though no enlarged lymph nodes may be felt on exam. Tumors greater than 4.0 mm. in thickness (thick lesions) have greater than 50% chance of spread to local lymph nodes and beyond. A second method of measuring tumor thickness is called the Clark’s level, which determines the level of the dermis (a part of the skin) that has been penetrated. The Clark’s level is used in conjunction with the thickness measurement to determine treatment.
Surgical treatment of melanoma centers around a wide local excision of the melanoma with a possible biopsy of one or more of the local lymph nodes. The treatment is guided by the thickness of the original lesion.
- Thin tumors (< 1.0 mm) can be treated by wide local excision alone with a greater than 90% cure rate. A 1 cm. margin all the way around the tumor is used to prevent recurrence. No lymph node biopsy is necessary.
- Intermediate thickness lesions (1.0 to 4.0 mm) or Clark’s level IV or greater require a 2-cm. margin for adequate excision. Lymph node biopsy is performed at the same surgery.
- Thick lesions (> 4.0 mm) require a 2 cm. or greater margin to prevent local recurrence. Lymph node biopsy is included if there is no evidence of distant disease (spread beyond the local lymph nodes) on X-ray imaging.
Additionally, a lymph node dissection (removal of all the closest lymph nodes) may be necessary if the patient has an enlarged lymph node that contains melanoma at the time of the original diagnosis, but no other identifiable distant disease. The goal is to remove all known disease to achieve a cure. However, it should be noted that a high percentage these patients may already have spread beyond these local lymph nodes, even though X-rays studies may suggest otherwise.
In the past, lymph node dissections were also performed on patients with intermediate thickness lesions in whom no enlarged lymph nodes could be felt. This was called an elective lymph node dissection. As mentioned above, 20% of these patients will have microscopic spread to the local lymph nodes that cannot be appreciated on exam. However, 80% of these patients would not benefit from this procedure since they have no lymph node spread at the time of diagnosis. This operation carries a 40% risk of lymphedema (chronic severe swelling of the affected extremity) as well as other risks. To identify patients with intermediate thickness lesions who would benefit from complete lymph node removal, a technique called lymphatic mapping and sentinel lymph node biopsy was developed. In this procedure, only a lymph node or two is removed, greatly decreasing the operative risks.
Lymphatic mapping is based on the theory that when a melanoma tumor spreads, it spreads first to the closest lymph node group. Within that group of lymph nodes, it spreads in an orderly fashion, so that there is one node that will become involved first: the sentinel lymph node. If this node can be identified and removed, the pathologist can tell if it contains metastatic melanoma cells. If it does, then that patient will likely benefit from a formal complete lymph node dissection. If it does not contain metastatic disease, then the patient has a < 2% chance of having melanoma in any of the remaining lymph nodes. In either case, patients require very close follow-up since most recurrences (local or distant) occur within the first 2 years.
The technique of lymphatic mapping begins in a radiology department prior to surgery where a radiologist injects a small amount of a radioactive marker into the skin adjacent to the melanoma site. In some cases, a scan called a lymphoscintigraphy is then performed, where a special machine is used to identify the lymph node region to which the melanoma drains. Some patients will not require this scan, depending on the site of the melanoma. Next, the patient is taken to surgery where the lymph node biopsy is performed. During the surgery, a blue dye may be injected into the skin adjacent to the melanoma site (some surgeons may opt to forgo the blue dye injection). In 5-10 minutes, the sentinel lymph node is stained blue. This dye, as well as the radioactive marker injected earlier by the radiologist, helps the surgeon identify the sentinel lymph node so that it can be removed. In 90% of the cases, the sentinel node can be found. In the other 10% of the cases, no lymph node biopsy is performed. In these patients, lymph node biopsies are performed when and if the patient develops an enlarged node.
A critical part of the post-surgical care of a melanoma patient is close follow-up. A patient with an intermediate thickness lesion or greater should be followed every 3-4 months for 2 years to monitor for local recurrence or development of distant disease. A dermatologist should also perform local skin checks at least every 6 months to monitor moles and hopefully prevent development of new melanomas.
Although complications from melanoma excisions with lymphatic mapping and sentinel lymph node biopsies are not common, they may occur. Complications may include the following:
- Bleeding or infection
- Melanoma recurrence
- Skin graft failures (if needed)
- Inability to find and biopsy a sentinel node
- Development of metastatic melanoma despite a negative sentinel node (false negative)
- Nerve injury (sensory or motor)
- Vascular injury