Your diagnosis

Squamous carcinoma in situ of the skin

This article will help you read and understand your pathology report for squamous carcinoma in situ of the skin.

by Jason Wasserman, MD PhD FRCPC, updated March 17, 2021

Quick facts:

  • Squamous carcinoma in situ (CIS) is a type of non-invasive skin tumour.
  • If left untreated, most tumours will eventually turn into an invasive type of cancer called squamous cell carcinoma.
  • Most tumours occur in older adults on sun-exposed skin usually as a result of the cells in the skin being damaged by UV light from the sun.
The 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.

What is squamous carcinoma in situ?

Squamous carcinoma in situ is a type of non-invasive skin cancer. Squamous carcinoma in situ starts from the squamous cells in the epidermis. Another name for squamous carcinoma in situ in the skin is Bowen’s disease. If left untreated, most tumours will eventually turn into an invasive type of cancer called squamous cell carcinoma.

What is the difference between squamous carcinoma in situ and squamous cell carcinoma?

In squamous carcinoma in situ, the tumour cells are seen only in the epidermis. In contrast, in squamous cell carcinoma, the tumour cells have broken through the basement membrane and entered the dermis below. The movement of tumour cells from the epidermis into the dermis is called invasion (see picture below).

Once tumour cells enter the dermis they are able to spread to other parts of the body such as lymph nodes. The movement of tumour cells from the tumour to a different part of the body is called metastasis.

squamous cell carcinoma and in situ

What causes squamous carcinoma in situ?

Most tumours that occur in older adults develop as a result of the cells in the epidermis being damaged by UV light from the sun. Long-term exposure to UV radiation from tanning beds can cause similar damage. People who have immunosuppression due to organ transplantation or HIV infection are also at increased risk of developing squamous carcinoma in situ.

How do pathologists make this diagnosis?

The diagnosis is usually made after a small tissue sample is removed in a procedure called a biopsy. 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.

What to look for in your report after the tumour has been removed

Margins

A margin is any tissue that was cut by the surgeon in order to remove the tumour from your body. Whenever possible, surgeons will try to cut tissue outside of the tumour to reduce the risk that any tumour cells will be left behind after the tumour is removed.

Margin

Your pathologist will carefully examine all the margins in your tissue sample to see how close the tumour cells are to the edge of the cut tissue. Margins will only be described in your report after the entire tumour has been removed.

A negative margin means there were no tumour cells at the very edge of the cut tissue. If all the margins are negative, most pathology reports will say how far the closest cancer cells were to a margin. The distance is usually described in millimetres.

A margin is considered positive when there are tumour cells at the very edge of the cut tissue. A positive margin is associated with a higher risk that the tumour will grow back (recur) in the same site after treatment.

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