Melanoma – Wide Local Excision

Pathology

Over the past several years, there has been a dramatic increase in the diagnosis of equivocal lesions whose biologic behavior cannot be predicted with absolute certainty. There is a spectrum that ranges from mild to severe dysplasia to atypical melanocytic proliferation to melanoma in situ to early invasive melanoma. Part of the increase in the incidence of melanoma almost certainly results from a lower threshold on the part of pathologists to diagnose such equivocal lesions as melanoma because of the potential consequences of a missed diagnosis of melanoma. It is now common for a pathology report to contain a long description that essentially states that the lesion may be anything from a severely dysplastic nevus to early invasive melanoma. In such cases, the prudent decision is to treat such lesions as an early invasive melanoma with a 1-cm margin WLE. Melanoma in situ is considered a premalignant precursor lesion that has a significant likelihood of progression to invasive melanoma. Because it does not invade beyond the basement membrane, it does not have access to blood vessels and lymphatics and does not generally have metastatic potential.

Histologically, invasive cutaneous melanoma is divided into four major types based on growth pattern and location. These forms are lentigo maligna melanoma, superficial spreading melanoma, acral lentiginous melanoma, and nodular melanoma. Melanomas arise as proliferations of melanocytes in the basal layer of the skin. As they multiply, these cells expand radially in the epidermis and superficial dermal layer, termed the radial growth phase. With time, the growth begins in a vertical direction as the skin lesion may become palpable, the so-called vertical growth phase. Nodular melanomas are an exception to this pattern, wherein the vertical growth phase is present early in tumor development. The vertical growth phase allows invasion into the deeper layers of the skin, where the tumor may achieve metastatic potential by invasion of blood and lymphatic vessels.

The histologic subtype of melanoma is not, in general, a major factor in prognosis; tumor thickness, ulceration, and other factors determine the prognosis. However, some histologic subtypes are more likely to be detected at a more advanced stage. Lentigo maligna melanoma occurs most commonly on the face of older individuals with sun-damaged skin and presents as a flat, dark, variably pigmented lesion, with irregular borders and a history of slow development ( Figure 53-1-3 ). Lentigo maligna melanomas may become large prior to diagnosis, because the slow progression may escape the patient's notice. Overall, the prognosis of lentigo maligna melanomas is better than for the other histopathologic types because of the often superficial nature of these tumors. However, lentigo maligna melanomas can pose challenging management problems because of their propensity to develop in cosmetically challenging areas (e.g., face), and the fact that the histologic extent of the lesion may extend well beyond the clinically apparent borders of the pigmented lesion. Thus, achieving negative margins may be challenging. Before embarking on complex tissue flaps for closure, it is prudent to ensure negative margins. This may necessitate delaying the closure until the final pathology report indicates negative margins of excision.

The most common histologic type is superficial spreading melanoma ( Figure 53-1-4 ). It is not necessarily associated with sun-exposed skin. As the name suggests, superficial spreading melanoma initially appears as a flat pigmented lesion growing in the radial dimension. If allowed to progress, these melanomas develop a vertical growth phase and invade more deeply into the skin.

Acral lentiginous melanoma (ALM) is classified by its anatomic site of origin. These tumors develop in the subungual areas, beneath the fingernails and toenails, and on the palms of the hand and soles of the feet ( Figure 53-1-5 ). This is the most common type of melanoma in black patients. The histologic appearance of ALMs is similar to melanomas arising on the mucous membranes. The diagnosis is often made at an advanced stage, which accounts for the poor prognosis of these tumors in general. Subungual acral lentiginous melanomas are often mistaken for subungual hematomas, leading to a delay in diagnosis. A key feature distinguishing a subungual melanoma from a subungual hematoma is that for subungual hematoma, the pigment should migrate distally with growth of the nail. Biopsy of subungual melanomas can be accomplished by performing a digital block with local anesthesia and removing the nail or performing a punch biopsy through the nail itself.

Nodular melanomas are raised papular lesions that develop a vertical growth pattern early in their course ( Figure 53-1-6 ). These melanomas often have a poor prognosis because of greater average tumor thickness and frequent ulceration.

Desmoplastic melanoma is a specific type of amelanotic melanoma, which commonly arises on the head and neck. Desmoplastic melanomas often exhibit neurotropism and have a greater propensity for local recurrence, with a decreased risk of nodal metastasis.