Melanoma In Situ of the Skin: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
April 23, 2026

Melanoma in situ is an early, non-invasive form of skin cancer that arises from melanocytes, the cells that produce melanin. In healthy skin, melanocytes sit in a neat row at the bottom of the epidermis — the thin outer layer of the skin. In melanoma in situ, these cells have become cancerous and have grown in abnormal numbers and patterns, but they are still confined to the epidermis. They have not yet broken through to the deeper layer of skin (the dermis), so they cannot spread to lymph nodes or other parts of the body.

Because the cancer cells remain in the epidermis, melanoma in situ is highly curable with appropriate surgery. However, if it is left untreated, it can progress to invasive melanoma, which has the potential to spread. This article will help you understand the findings in your pathology report for melanoma in situ of the skin — what each term means and why it matters for your care.

What causes melanoma in situ?

Most melanomas in situ are caused by damage to the DNA in melanocytes from ultraviolet (UV) radiation. Most of this damage comes from the sun, but artificial sources such as tanning beds cause similar injury. Over many years, UV exposure leads to genetic changes in melanocytes that allow them to grow uncontrolled. A smaller number of melanomas in situ arise without a clear link to sun exposure.

Other risk factors include:

• Fair skin — People with lighter skin, light-colored eyes, freckles, and red or blond hair are at higher risk because their skin contains less protective melanin.
• Many or unusual moles — A large number of moles, or moles that look atypical (dysplastic), are associated with a higher lifetime risk of melanoma.
• Older age — The risk increases with age as UV damage accumulates.
• Personal or family history — A previous melanoma or a close relative with melanoma substantially increases the risk.
• Weakened immune system — People taking anti-rejection medications after an organ transplant or living with other forms of immune suppression have a higher risk.
• Inherited conditions — Rare inherited mutations, such as in the CDKN2A or CDK4 genes, are responsible for a small percentage of melanomas and may run in families.

What are the symptoms of melanoma in situ?

Melanoma in situ usually appears as a flat or slightly raised patch on the skin. It is most common on sun-exposed areas such as the head, neck, trunk, arms, and legs, but it can occur anywhere on the body. Common features include:

• Irregular shape and border — The edges are often uneven or poorly defined, and it may be difficult to tell where the lesion ends and the normal skin begins.
• Mixed colors — The lesion may contain several shades of brown or black, sometimes with lighter or reddish areas.
• Slow growth — Melanoma in situ often grows slowly over months to years.
• No pain or bleeding — Melanoma in situ is usually painless and does not typically itch, bleed, or ulcerate. These features raise concern for invasive melanoma instead.

Because melanoma in situ can resemble a harmless mole or sunspot, it is often detected because it changes over time, looks different from other spots on a person’s body, or is found during a routine skin examination. Any new, changing, or unusual spot should be assessed by a healthcare professional.

How is the diagnosis made?

The diagnosis of melanoma in situ is made after a tissue sample is examined under the microscope by a pathologist. The sample is obtained by a skin biopsy. For a lesion suspected to be melanoma, an excisional biopsy — in which the entire lesion is removed with a narrow margin of surrounding skin — is preferred whenever possible because it allows the pathologist to examine the whole lesion and confirm that no invasive cancer is hidden within it. Punch or shave biopsies may be used when an excisional biopsy is not practical because of the size or location of the lesion.

Under the microscope, the pathologist looks for abnormal melanocytes that are larger, darker, and more irregular than normal melanocytes. These cells grow in increased numbers along the bottom of the epidermis, often extending upward into the middle and upper layers (a pattern called pagetoid spread) and spreading along the walls of hair follicles. Crucially, the abnormal cells remain entirely within the epidermis — none have crossed into the dermis. The surrounding skin typically shows changes of long-term sun damage, including solar elastosis. In difficult cases, immunohistochemistry is used to confirm the diagnosis. Markers such as SOX10, Melan-A, and PRAME highlight the melanocytes and help the pathologist map the full extent of the disease, particularly along the edges of the lesion.

Melanoma in situ and lentigo maligna

Lentigo maligna is a specific type of melanoma in situ that develops on skin with severe, long-standing sun damage — most often on the face, neck, and backs of the hands of older adults. Under the microscope, lentigo maligna shows many of the same features as other melanomas in situ, but the background skin typically shows extensive solar elastosis, and the epidermis is often thinned. If left untreated, lentigo maligna can progress to an invasive form called lentigo maligna melanoma. Because the borders of lentigo maligna are often poorly defined both clinically and under the microscope, achieving a clear surgical margin can be more challenging than with other types of melanoma in situ.

Margins

A margin is the edge of the tissue removed during surgery. Pathologists examine margins under the microscope to see whether any cancer cells are present at the cut edge. For melanoma in situ, both the peripheral margin (the side edges of the removed skin) and the deep margin (the bottom of the removed tissue) are evaluated.

• Negative margin — No cancer cells are seen at the cut edge. The tumor is considered completely excised, and the risk of recurrence is low.
• Close margin — Cancer cells are near the cut edge but do not reach it. Many reports describe the distance (in millimeters) from the tumor to the margin. A close margin may prompt a second, wider surgery, particularly for lentigo maligna and other lesions with indistinct borders.
• Positive margin — Cancer cells are present at the cut edge. Additional surgery is usually recommended to ensure that all abnormal cells have been removed.

When melanoma in situ is diagnosed on a small biopsy, the sample is almost always expected to show positive margins because the biopsy was intended to diagnose the lesion, not remove it completely. In this situation, a wide local excision is performed afterward to remove the remaining tumor with a clear margin of healthy tissue.

What is the prognosis?

The prognosis for melanoma in situ is excellent. Because the cancer cells are confined to the epidermis and cannot reach the blood vessels or lymphatic channels in the dermis, melanoma in situ cannot spread to lymph nodes or distant parts of the body. When it is completely removed, it is considered cured, and the long-term survival rate is essentially the same as for people who have never had melanoma.

The main risks after a diagnosis of melanoma in situ are:

• Local recurrence — A small number of melanomas in situ return at the same site, most often when the original margins were close or positive. Lentigo maligna has a higher recurrence rate than other types of melanoma in situ because its borders are often poorly defined.
• A new melanoma at a different site — People who have had one melanoma are at increased risk of developing another melanoma or other skin cancers elsewhere on the body, which is why ongoing skin surveillance is important.
• Hidden invasive disease — Occasionally, a small area of invasive melanoma is present within a lesion diagnosed on a small biopsy as melanoma in situ. This is one of the reasons that excising the whole lesion and examining it thoroughly is important.

What happens after the diagnosis?

Treatment of melanoma in situ is usually coordinated by a dermatologist and, in some cases, a surgeon. The primary treatment is surgical removal.

For most melanomas in situ, a wide local excision is performed to remove the entire lesion with a margin of healthy tissue around it — typically 5 to 10 millimeters, depending on the location and subtype. For lentigo maligna and other lesions with poorly defined borders on the face or other cosmetically sensitive areas, Mohs micrographic surgery or a staged excision may be preferred. These techniques allow the surgeon to check the margins carefully during or between surgeries and to preserve as much healthy tissue as possible.

In selected situations — for example, lentigo maligna in an older patient who is not a good surgical candidate, or for lesions in areas where surgery would be disfiguring — non-surgical options may be considered. These include the topical medication imiquimod (which activates the immune system to attack the abnormal cells) and radiation therapy. These approaches are less effective than surgery and are typically reserved for carefully selected patients.

After treatment, regular skin examinations are important. People who have had one melanoma in situ are at increased risk of developing additional melanomas or other skin cancers, and consistent sun protection — including sunscreen, sun-protective clothing, and avoiding tanning beds — is a key part of prevention.

Questions to ask your doctor

• Where on my body did the melanoma in situ develop?
• Was the specific type of lentigo maligna diagnosed, or was it another type of melanoma in situ?
• Was any invasive melanoma found in the biopsy, or only in situ disease?
• Were the surgical margins negative, close, or positive?
• Do I need a wide local excision, and how much additional skin will be removed?
• If the lesion is on my face or another cosmetically sensitive area, is Mohs surgery or staged excision appropriate?
• Are non-surgical options (such as imiquimod or radiation therapy) being considered, and why?
• What is my risk of the melanoma coming back at the same site?
• Am I at higher risk of developing additional melanomas or other skin cancers? How often should I have full skin examinations?
• Should my family members be screened or evaluated for inherited melanoma risk?
• What steps can I take to reduce my risk of future skin cancers?