Malignant Melanoma of the Head and Neck: A Brief Review of Pathophysiology, Current Staging, and Management

Melanocyte Physiology

Melanoma is a malignant tumor of melanocytes, neural crest derivatives that contain melanin-producing organelles known as melanosomes. Melanocytes are normally found in the basal layer of the epidermis but are also found in the eye, gallbladder, anus, and vagina, and throughout the upper aerodigestive tract. In the skin, there is an extensive interaction between the melanocytes and the adjacent keratinocytes, as melanin is continuously transferred from the melanocytes to the keratinocytes as a result of stimulation by ultraviolet radiation.1

Background and Epidemiology

The incidence of malignant melanoma has steadily increased over the last 40 years at a rate of approximately 5% annually. The current worldwide incidence is 10.9 per 100 000 people. Among Americans, the current lifetime risk of developing melanoma is 1 in 75. Although the overall melanoma-specific survival has increased in the last 20 years, the prognosis for those with advanced disease is no better than it was 20 years ago.2

Risk Factors

Those at greatest risk for developing melanoma are fair-skinned individuals, especially redheads. The risk of developing melanoma is heightened in those living in climates with intense sunlight. Sun exposure is the single greatest risk factor for developing melanoma as both the A and B types of ultraviolet rays are implicated in development of the disease. Patients with pigmented lesions such as junctional nevi, dysplastic nevi, congenital nevi greater than 20 cm in greatest dimension, and those with large numbers of benign nevi are at increased risk for the development of melanoma. Rare congenital syndromes such as dysplastic nevus syndrome (familial atypical mole-melanoma syndrome) and xeroderma pigmentosa carry a nearly 100% risk of developing melanoma. Any childhood sun exposure with sunburn heightens one's risk.

Interestingly, the current thought regarding the relationship between sun exposure and melanoma development favors less frequent, more intense sun exposure as a stronger risk factor than chronic sun exposure. This assertion is based on the hypothesis that those with chronic, in some cases daily, sun exposure compensate for the solar insult in a way that those with less frequent exposure fail to do.2–4

Clinical Presentation

The hallmark of malignant melanoma is a pigmented lesion that displays the often-cited ABCDE of melanoma: A refers to asymmetry of the lesion, B to border irregularity, C to variegated color, D to diameter greater than 6 mm, and E to elevation. One should also take the time to evaluate the surrounding skin for recent changes and elicit subjective symptoms such as pain or itching at the lesion site. It is also important to document ulceration of the lesion prior to biopsy as ulceration plays a critical role in the current TNM staging system. Finally, one should be mindful that not all melanoma lesions are pigmented. Up to 10% of melanoma lacks melanin and, rarer still, the pigmented melanoma lesion may spontaneously regress leaving a hypopigmented “halo.”

Diagnosis

The differential diagnosis of melanoma is vast including an array of benign, sun-induced, vascular, inflammatory, and neoplastic lesions. Because of this overlap and, more importantly, because of the possibility of failing to diagnose a potentially lethal disease, one's threshold for biopsy should be low. As previously stated, one must be sure to document ulceration prior to biopsy. The preferred method of tissue sampling for the diagnosis of melanoma is excisional biopsy with a margin of normal tissue measuring at least 2 mm. It is, however, acceptable to perform an incisional biopsy in larger lesions or in those in which an excisional biopsy would violate critical structures such as the eyelid or oral commissure. One should never, though, perform a shave or needle biopsy of a suspected melanoma. Once the diagnosis of melanoma has been clinched, work-up should progress based on the clinical stage of the disease.

Melanoma Growth Patterns

The natural history of melanoma is characterized by two distinct growth phases: radial (intraepithelial) and vertical (intradermal). The radial growth phase is circumferential in nature, growing within the epidermis to, but not beyond, the dermal-epidermal junction.5,6 At some point in the radial growth phase, the tumor undergoes a clonal change that confers a survival advantage that allows it to enter the vertical growth phase as it penetrates the dermal-epidermal junction and grows into and beyond the dermis. These two well-described phases serve as the basis for the two classic staging systems for melanoma, those described by Clark and Breslow (Table 1).7,8

Melanoma Subtypes

Staging

Classically, melanoma lesions have been staged according to the Clark staging system, a description of the histologic level of invasion, and the Breslow staging system, a measure of the absolute depth of the lesion.7,8 With the adoption of the American Joint Committee on Cancer (AJCC) TNM staging system (Figure 1),9 the definition of nodal staging has become increasingly important. Between 10% and 30% of melanoma patients present with clinically detectable cervical metastases. Interestingly, in 55% of patients, the observed drainage patterns are different than those predicted based on the sight of the primary tumor. The strongest predictor of the presence of regional metastases is the thickness of the primary lesion with 65% of those with lesions greater than or equal to 4 mm in thickness presenting with cervical metastases.

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Figure 1 American Joint Committee on Cancer staging for melanoma.9

Any discussion of nodal disease in melanoma would be remiss without defining the terms satellite lesions and in-transit lesions. Satellite lesions refer to foci of melanoma found within 2 cm of the primary tumor. They are seen in 3% of patients. In-transit lesions are foci of melanoma found greater than 2 cm beyond the primary tumor but not beyond the predicted first echelon of lymph nodes draining the primary tumor. Both satellite and in-transit lesions play a role in the current TNM staging system.

Current modalities for evaluation of regional nodal metastases include computed tomography, magnetic resonance imaging, positron emission tomography, and lymphoscintigraphy. Currently, lymphoscintigraphy coupled with sentinel node biopsy is considered the “gold standard” for evaluation of nodal disease in the clinically negative neck. It should be remembered, though, that the lymphoscintigram is merely a snapshot of the dynamic drainage patterns of the head and neck. As previously discussed, lymphatic drainage patterns of melanoma lesions vary from patient to patient. Willis and Ridge, though, asserted that the same discordance exists within individual patients.10

Consequently, one should bear in mind that a single lymphoscintigraphy study may not capture all affected nodal basins. This fact and its implications gain added gravity when one recalls the finding of Balch et al that the presence or absence of nodal metastases is the single most important prognostic factor in the survival of melanoma patients.11

Management of the Neck

The management of the clinically negative neck in melanoma patients has been the subject of some debate in recent years. The current recommendations favor observation in those with primary lesions less than 1 mm in thickness, Clark's level less than IV, and without palpable cervical lymphadenopathy. For all other patients, the options include elective neck dissection and sentinel node biopsy. In a retrospective study, Balch and colleagues demonstrated a significant survival advantage in performing elective neck dissection in those with intermediate thickness (1–4 mm) melanoma.12–15 The prospective Intergroup Melanoma Surgical Trial narrowed Balch's recommendations to those with stage II and III disease, who are less than 60 years of age, with lesions 1 to 2 mm in thickness.16 In a 2002 meta-analysis, however, Lens et al scrutinized the three largest prospective trials on the topic and found no survival benefit in those with intermediate-thickness lesions.17

The current standard in the diagnosis of cervical metastases in those with intermediate to thick lesions is sentinel node biopsy. The use of this technique was initially shown to improve survival in patients with infraclavicular melanoma. Gershenwald and colleagues found the status of the sentinel node to be the strongest predictor of disease-free survival in melanoma patients. Several other studies have demonstrated the strong negative predictive value of a negative sentinel node biopsy and the benefit of expediting a formal neck dissection following a positive sentinel node biopsy.18–21

Comprehensive Management

The cornerstone of melanoma management is excision with appropriate deep and peripheral margins. The depth of the excision should include the underlying subcutaneous tissue to the level of the deep fascia. Historically, in order for the excision to be considered “oncologically sound,” the peripheral margins should measure at least 5 cm. Today, more conservative recommendations prevail (Table 2). When considering surgical margins, one should be wary of frozen section evaluation of melanoma. The disease is not readily identified with this technique and is, thus, not amenable to Mohs micrographic surgery.

Formal cervical lymphadenectomy is recommended in patients with a clinically positive neck or in those with a positive sentinel node biopsy. The extent of neck dissection is tailored to the site of the primary tumor. Radical neck dissection has classically been considered the “gold standard” in the management of the melanoma-infiltrated neck. Studies have indicated less frequent recurrence among those undergoing modified radical neck dissection compared to those undergoing radical neck dissection.22 Though there are little prospective data on the topic, the experience with modified radical neck dissection has been extrapolated to selective neck dissection and this procedure is commonly performed.

Melanoma is traditionally considered a radioresistant tumor, though some studies support the use of hypofractionated radiation as an adjunct to surgery.23 At this time, radiation is reserved for adjuvant treatment following definitive surgical treatment and as a primary treatment in those who are poor surgical candidates, with extensive facial lentigo maligna melanoma, neurotropic lesions, extracapsular spread, multiple node involvement (>4), or recurrence.24

Systemic chemotherapy or immunotherapy is reserved for those who have completed locoregional therapy and are at high risk for recurrence. These patients include those with ulcerated primary lesions, lesions greater than 4 mm in thickness, and those with in-transit, nodal, or distant metastases. Currently, systemic chemotherapy plays a principally palliative role in the treatment of melanoma.

The wave of the future in systemic therapy for melanoma is immunotherapy. Immunomodulators such as interferon-alpha are already in use. These factors work by attempting to induce an immune response to the tumor leading to spontaneous regression. Meta-analysis data suggest that biochemotherapy with interferon and the cytokine interleukin-2 coupled with standard chemotherapeutic regimens may improve response rates but does not affect overall survival.25