Squamous cell carcinoma of the skin

What is squamous cell carcinoma?

Squamous cell carcinoma (SCC) is a very common type of skin cancer. It develops from specialized skin cells called squamous cells. 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 cell carcinoma.

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.

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.

squamous cell carcinoma and in situ


Pathologists use the word grade to describe the difference between the cancer cells in squamous cell carcinoma and the normal, healthy squamous cells in the skin. The tumour grade is important because poorly differentiated tumours tend to grow faster and are more likely to spread to lymph nodes or other parts of the body.

The grade is divided into three levels of differentiation based on how the cancer cells look when examined under the microscope:

  1. Well differentiated – The cancer cells in this tumour look very similar to normal squamous cells.
  2. Moderately differentiated – The cancer cells in this tumour are abnormal but they still resemble squamous cells.
  3. Poorly differentiated – The cancer cells in this tumour look very little or nothing like normal squamous cells. Additional tests such as immunohistochemistry may be needed to prove the tumour is a squamous cell carcinoma.

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

Tumour size

This is the size of the tumour. The tumour is usually measured in three dimensions but only the largest dimension is described in your report. For example, if the tumour measures 4.0 cm by 2.0 cm by 1.5 cm, your report will describe the tumour as being 4.0 cm.

Depth of invasion (tumour thickness)

All squamous cell carcinomas start in the epidermis on the outer surface of the skin. Depth of invasion describes how far the cancer cells have travelled from the epidermis into the tissue below. The movement of cancer cells from the epidermis into the tissue below is called invasion.

The depth of invasion is measured from the surface of the skin to the deepest point of invasion. Some pathology reports describe the depth of invasion as tumour thickness. Tumours that grow deeper into the dermis are more likely to spread to a lymph node or to grow back after treatment. The movement of cancer cells to a lymph node or another part of the body is called metastasis.

Perineural invasion

Nerves are like long wires made up of groups of cells called neurons. Nerves send information (such as temperature, pressure, and pain) between your brain and your body. Perineural invasion means that cancer cells were seen attached to a nerve.

Cancer cells that have attached to a nerve can use the nerve to travel into tissue outside of the original tumour. This increases the risk that the tumour will come back in the same area of the body (recurrence) after treatment.

perineural invasion

Lymphovascular invasion

Blood moves around the body through long thin tubes called blood vessels. Another type of fluid called lymph which contains waste and immune cells moves around the body through lymphatic channels. Cancer cells can use blood vessels and lymphatics to travel away from the tumour to other parts of the body. The spread of cancer cells from the tumour to another part of the body is called metastasis.

Before cancer cells can metastasize, they need to enter a blood vessel or lymphatic. This is called lymphovascular invasion. Finding lymphovascular invasion in a tumour is important because it increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as the lungs.

lymphovascular invasion

High-risk features

A high-risk feature is something that makes the tumour more likely to come back in the future, either at the same site or to spread to a distant site such as the lungs.

Pathologists look for the following high-risk features in squamous cell carcinoma of the skin:

  • A depth of invasion of more than 2 millimetres.
  • The presence of cancer cells in the lower half of the skin.
  • Perineural invasion (see section above).
  • Lymphovascular invasion (see section above).
  • Poorly differentiated tumours (see Histologic grade section above).

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 cancer 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 cancer 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 cancer 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.


Lymph nodes

Lymph nodes are small immune organs located throughout the body. Cancer cells can travel from the tumour to a lymph node through lymphatic channels located in and around the tumour (see Lymphovascular invasion above). The movement of cancer cells from the tumour to a lymph node is called metastasis.

Lymph nodes are not typically removed for squamous cell carcinoma of the skin. However, if lymph nodes are removed your pathologist will carefully examine each one of them for cancer cells. Lymph nodes that contain cancer cells are often called positive while those that do not contain any cancer cells are called negative. Most reports include the total number of lymph nodes examined and the number, if any, that contain cancer cells.

Lymph node

Tumour deposit

A group of cancer cells inside of a lymph node is called a tumour deposit. If a tumour deposit is found, your pathologist will measure the deposit and the largest tumour deposit found will be described in your report. Larger tumour deposits are associated with a worse prognosis.

Extranodal extension

All lymph nodes are surrounded by a capsule. Extranodal extension means that cancer cells have broken through the capsule and into the tissue that surrounds the lymph node. Extranodal extension is also associated with a higher risk of new tumours developing in the neck and is often used by your doctors to guide your treatment.

extranodal extension

by Allison Osmond, MD FRCPC (updated August 18, 2021)
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