Squamous carcinoma in situ of the oral cavity

What is squamous carcinoma in situ (CIS)?

Squamous carcinoma in situ is a type of non-invasive cancer. The tumour starts from the squamous cells on the inner surface of the oral cavity. Another name for squamous carcinoma in situ is severe squamous dysplasia.

If left untreated, squamous carcinoma in situ almost always turns into a type of invasive cancer called squamous cell carcinoma. Squamous carcinoma in situ can start in any part of the oral cavity although the most common location is the lateral border of the tongue.

The oral cavity

The oral cavity is the beginning of the aerodigestive tract. The oral cavity starts at the lips and ends where the mouth meets the throat. The oral cavity is made up of multiple parts including the lips, tongue, floor of mouth, gingiva, buccal mucosa, hard palate, and retromolar trigone.

Oral cavity

The inner surface of the oral cavity is covered by cells called squamous cells. that form a barrier called the epithelium. The tissue below the epithelium is called the stroma. Pathologists use the word mucosa to describe tissue that includes both the epithelium and the stroma.

Oral Cavity Normal

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

The abnormal cells in squamous carcinoma in situ are only seen in the epithelium. This is different from squamous cell carcinoma where the abnormal cells are also seen in the stroma below the epithelium. The movement of abnormal cells from the epithelium into the stroma is called invasion. Your pathologist will carefully examine your tissue sample to make sure there is no evidence of invasion before making the diagnosis of squamous carcinoma in situ.

squamous carcinoma in situ oral cavity

What causes squamous carcinoma in situ in the oral cavity?

The most common cause of squamous carcinoma in situ in the oral cavity is smoking. Other causes include excessive alcohol consumption, immune suppression, and prior radiation to the head and neck.

How do pathologists make this diagnosis?

The diagnosis of squamous carcinoma in situ is usually made after a small sample of tissue is removed in a procedure called a biopsy. The biopsy is usually performed because you or your doctor saw an abnormal-looking area of tissue within your oral cavity. Your pathology report will probably say what part of the oral cavity was sampled in the biopsy.

The diagnosis of squamous carcinoma in situ can only be made after a tissue sample is examined under the microscope. Compared to normal, healthy squamous cells, the abnormal cells in an area of squamous carcinoma in situ are usually larger and darker. Pathologists use the word hyperchromatic to describe cells that look darker than normal cells. Large clumps of genetic material called nucleoli may also be seen in the nucleus of the abnormal cells. These squamous cells also commonly undergo an abnormal pattern of development which results in a process called keratinization.

What to look for in your report after the tumour is removed

Tumour size

This is the size of the tumour measured in centimetres. The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 2.0 cm by 1.5 cm by 1.2 cm, your report will describe the tumour as being 2.0 cm.


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 cancer 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 tumour 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 come back (recur) in the same site after treatment.


by Jason Wasserman, MD PhD FRCPC (updated August 23, 2021)
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