Squamous carcinoma in situ (skin)

What is squamous carcinoma in situ?

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

What is 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 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.

When examined under the microscope, the tumour cells in squamous cell carcinoma in situ are larger and darker than normal squamous cells. Pathologists use the terms atypical or atypia to describe cells that look different than normal, healthy cells. Unlike invasive squamous cell carcinoma, all of the tumour cells in squamous cell carcinoma in situ are seen in the epidermis. Tumour cells that are found only in the epidermis are unable to spread to other parts of the body.

squamous cell carcinoma and in situ

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


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.

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.


by Jason Wasserman, MD PhD FRCPC (updated August 18, 2021)
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