Melanoma in situ of the skin

What is melanoma in situ?

Melanoma in situ is a type of early, non-invasive skin cancer. The tumour is made up of specialized cells called melanocytes. Another name for melanoma in situ is lentigo maligna. In melanoma in situ, the abnormal melanocytes are only found in the top layer of the skin called the epidermis. If left untreated, this tumour can change over time into a type of cancer called invasive melanoma.


Skin is made up of three layers: epidermis, dermis, and subcutaneous fat. The surface and the part you can see when you look at your skin is called the epidermis. The cells that make up the epidermis include squamous cells, basal cells, melanocytes, Merkel cells, and cells of the immune system. The squamous cells in the epidermis produce a material called keratin which makes the skin waterproof and strong and protects us from toxins and injuries.

The dermis is directly below the epidermis. The dermis is separated from the epidermis by a thin layer of tissue called the basement membrane. The dermis contains blood vessels and nerves. Below the dermis is a layer of fat called subcutaneous adipose tissue.


Melanocytes are small cells that produce melanin, a dark pigment that helps protect our skin from the sun’s ultraviolet rays. The amount of melanin normally found in our skin determines our skin colour. People with very light skin produce very little melanin while people with very dark skin produce a lot of melanin.

How do pathologists make this diagnosis?

Without a microscope, melanoma in situ can look like a dark brown or black growth on the skin. The growth usually has an irregular shape and it may be difficult to see where the growth ends and the normal skin begins (the border).

When examined without a microscope, melanoma in situ can look very similar to a type of cancer called invasive melanoma. The only way to tell the difference between these two conditions is for your doctor to remove a small sample of tissue in a procedure called a biopsy. Your pathologist will then examine the tissue under the microscope and make a diagnosis.

The diagnosis can also be made after the entire tumour is removed in a procedure called an excision. If the diagnosis is made after a biopsy, your doctor will probably recommend a second surgical procedure to remove the rest of the tumour.

When examined under the microscope, the melanocytes in melanoma in situ are larger and darker than normal melanocytes and they grow in groups or in between the keratinocytes in the epidermis. If these abnormal melanocytes move beyond the epidermis into the dermis or subcutaneous tissue the diagnosis changes to invasive melanoma. The movement of tumour cells from the epidermis into the dermis is called invasion.


melanoma in situ


A margin is any tissue that was cut by the doctor in order to remove the abnormal tissue from your body. If all of the abnormal tissue has been removed, your report will say that the margins are negative. In contrast, a positive margin or “incompletely excised” means that some of the abnormal tissue has been left in your body. In that case, your doctor will probably suggest another surgical procedure to remove the rest of the abnormal tissue.


by Robyn Ndikumana MD BScN and Allison Osmond, MD FRCPC (updated August 19, 2021)
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