Melanoma in Situ (Stage 0 Melanoma)

What "in situ" means

"In situ" is Latin for "in its original place." In melanoma in situ, the abnormal melanocytes (pigment cells) are confined to the epidermis and have not crossed the basement membrane into the dermis. Because there are no blood or lymph vessels in the epidermis, the cells cannot spread to lymph nodes or other organs.

Once melanoma invades the dermis, it becomes invasive melanoma (stage 1 or higher), and thickness (Breslow depth) determines prognosis. See melanoma for invasive disease.

What melanoma in situ looks like

Dermatologists use the ABCDE rule:

• Asymmetry: one half does not match the other
• Border: irregular, notched or blurred
• Color: several shades of brown, black, red, blue or white
• Diameter: larger than about 6 mm, though early melanomas can be smaller
• Evolving: changing in size, shape, color or symptoms

The ugly duckling sign, a spot that looks different from your other moles, is also important.

Lentigo maligna

Lentigo maligna is a subtype of melanoma in situ on chronically sun-damaged skin, usually the face of older adults. It appears as a slowly enlarging, flat, irregular tan to brown patch that may grow over years. It can be hard to distinguish from sunspots and often extends beyond its visible edges.

Risk factors

• many moles, or atypical (dysplastic) moles
• fair skin, light eyes, red or blond hair, freckling
• history of sunburns, especially in childhood, and tanning bed use
• personal or family history of melanoma
• weakened immune system
• older age for lentigo maligna

Diagnosis

1. skin examination with dermoscopy
2. biopsy: ideally removing the whole lesion with a narrow margin, so the pathologist can confirm it is in situ and not invasive
3. pathology report: states "melanoma in situ" and whether margins are clear

Imaging and lymph node biopsy are not needed for melanoma in situ.

Treatment

Wide local excision

The standard treatment is surgically removing the melanoma with a margin of normal-looking skin, typically about 0.5 to 1 cm. The wound is closed with stitches. This is done under local anesthetic in a clinic or office.

Lentigo maligna

Because it often extends beyond the visible edge, options include:

• Mohs surgery or staged excision, which check all margins under the microscope
• imiquimod cream or radiation therapy for people who cannot have surgery, though recurrence rates are higher

Outlook and follow-up

Melanoma in situ treated with complete excision has an excellent prognosis: survival is essentially the same as people without melanoma. Recurrence at the same site is uncommon with clear margins but is more likely with lentigo maligna.

People who have had melanoma in situ have a higher risk of developing a new melanoma elsewhere, so:

• see a dermatologist for skin checks, typically every 6 to 12 months for a few years, then yearly
• check your own skin monthly, including scalp, soles, between toes and nails
• protect your skin from the sun: SPF 30 or higher, hats, clothing, shade, no tanning beds
• consider skin checks for close family members

See skin cancer screening for how to check your skin.

Frequently asked questions

Is melanoma in situ cancer?

Yes, it is the earliest stage of melanoma, called stage 0. The cancerous cells are confined to the top layer of skin and have not invaded deeper tissue, so they cannot spread. Complete removal is almost always curative.

How is melanoma in situ treated?

It is treated with surgical excision, removing the melanoma with a margin of about 0.5 to 1 cm of normal skin. Lentigo maligna on the face may be treated with Mohs surgery or staged excision. Chemotherapy and radiation are not usually needed.

Can melanoma in situ spread?

No, not while it is truly in situ, because it has not reached the blood and lymph vessels in the deeper skin. If left untreated, it can become invasive and then may spread, which is why prompt removal matters.

Can melanoma in situ come back?

Recurrence at the same site is uncommon after removal with clear margins, but more common with lentigo maligna. People who have had it are more likely to develop a new melanoma elsewhere, so regular skin checks are important.

What is the survival rate for stage 0 melanoma?

Survival is close to 100 percent. Stage 0 melanoma confined to the epidermis is cured by complete excision in nearly all cases. The main ongoing risk is developing a separate new melanoma later.