Melanoma in situ - Skin -

This article will help you read and understand your pathology report for melanoma in situ.

by Robyn Ndikumana MD BScN and Allison Osmond, MD FRCPC,  updated on June 2, 2020

Quick facts:
  • Melanoma in situ is an early form of non-invasive skin cancer and a type of melanoma.
  • Unlike other types of melanoma, the cancer cells in melanoma in situ have not spread beyond the surface of the skin.
  • It develops in sun damaged skin and is most common on the head and neck.
The anatomy and histology of skin

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

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.

What is melanoma in situ?

Lentigo maligna is a type of early, non-invasive skin cancer made up of 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.

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).

Unfortunately, melanoma in situ and malignant melanoma can look the same without a microscope. 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.

How do pathologists make this diagnosis?

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 malignant melanoma. The movement of tumour cells from the epidermis into the dermis is called invasion.

 

melanoma in situ

Margins

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.

Margin

Treatment

If the area of melanoma in situ involves a large area of skin, you may be offered a cream called Imiquimod (Aldera). This is particularly useful for cosmetically sensitive areas like the face.

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