Skin Cancer Types
Superficial Spreading Melanoma (SSM)
Superficial spreading melanoma (SSM), also known as Clark melanocytic nevus, accounts for about 70 percent of melanomas. It usually appears after puberty and is the most common type in young people. In women, this type of melanoma often occurs on the sun-exposed surface of the skin over the shin bone, between the knees and the ankles. In men, SSM usually develops on the back of the torso and along the front of the legs. SSM of the lower extremities also may develop on the soles of the feet after an injury.
Superficial spreading melanoma usually develops from a preexisting mole (nevus). It may grow slowly within the upper skin before extending into the deeper skin layers. This melanoma often appears as a single flat or slightly raised "spot" with irregular borders. It may contain areas of tan, brown/black, red, dark blue/purple, or white coloration.
SSM undergoes two growth phases. During the "radial" phase, the lesion expands through the epidermis (upper skin layer). In the early radial phase, which may last months or years, the lesion is thin. After the radial phase, a "vertical" growth phase begins. Once the melanoma grows into the dermis (lower skin) and underlying structures, it can invade other tissues and the prognosis worsens.
Nodular Melanoma (NM)
Nodular melanoma (NM) usually appears as a uniformly-colored black nodule (small, knot-like bump). It also may be brown, blue, gray, white, tan, or red in color. The nodule feels round to the touch, and the borders are smooth and very regular. These lesions often are mistaken for blood vessel abnormalities.
Nodular melanoma is the most aggressive type and the second most common melanoma. NM usually develops in unblemished skin, rather than from a preexisting mole. In many cases, this type is invasive at the time of the diagnosis. NM usually affects light-skinned individuals in areas of frequent sun exposure (e.g., arms, legs, head, neck, scalp). It is most common in middle-aged people and has a higher incidence in Japanese populations.
Acral Lentiginous Melanoma (ALM)
This type of malignant melanoma is characterized microscopically by the arrangement of large melanocytes (pigment-producing cells) with multiple or small lobe (lobulated) cell centers in a single file, or "lentiginous" pattern.
Acral lentiginous melanoma (ALM) is more common in Asians and African Americans than in Caucasians. It accounts for 2% to 8% of melanomas in Caucasians and up to 60% of melanomas in dark-skinned populations. It is more prevalent in patients over the age of 60.
ALM commonly occurs in the areas around the nails, the palms of the hands, and the soles of the feet. Because it may develop under the nail beds, treatment is often delayed, resulting in a poor prognosis.
ALM may resemble a bruise, blood blister, or brown-black blotch on the toe or finger. This type usually is a slow-growing lesion that spreads along the surface before penetrating the skins lower layers. ALM may involve part of the nail or the entire nail and surrounding tissue. The lesions may have a streaked appearance, with parts having well defined borders and others having a blurred appearance. The borders often are highly irregular. Traumatic injury to a mole may stimulate a transformation into a malignancy.
Lentigo Maligna (LM)
Lentigo maligna tends to spread slowly along the surface layers of the skin and may appear as a flat or slightly raised spot. This type can be mottled tan, brown, or dark brown/black in color. LM usually does not metastasize. It typically is found in older people who have sun-damaged skin on the face, ears, arms, or upper body (e.g., lentigo maligna is prevalent in residents of Hawaii). This type of melanoma represents about 5 percent of all cases. If lentigo maligna becomes invasive, it is called lentigo maligna melanoma (LMM).
Many noncancerous (benign) skin lesions resemble malignant melanomas. These usually are chronic conditions, but they require monitoring because they may have the potential to become cancerous. For example, dysplastic nevus syndrome has a high risk for transforming into superficial spreading melanoma (SSM). An individual with this condition will have many moles of a uniform, dark color.
Seborrheic keratosis is a condition in which lesions are raised above the skin surface and are dark yellow/brown in color and waxy. They are oval-shaped and larger than the moles of dysplastic nevus syndrome. Seborrheic keratoses are not painful, but they may occur on pressure points on the bottom of the foot. They arise from the upper layer (epidermal) of skin cells, are noncancerous (benign), and do not spread.
Certain moles (nevi) occur as solitary benign lesions. A junctional nevus is a flat, light brown, symmetrical mole. When this mole is viewed under the microscope, groups of melanin-containing nevus cells are seen at the junction between the dermal and epidermal skin layers. These moles can be removed if they are extremely large. A compound nevus is small, round, and elevated (papular), dark in color, and has a soft texture.