Skin -

Squamous cell carcinoma

This article was last reviewed and updated on August 9, 2019
by Allison Osmond, MD FRCPC

Quick facts:

  • Squamous cell carcinoma is a type of skin cancer.

  • It starts from the cells on the surface of the skin. 

  • 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. 

Normal 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 cell carcinoma?

Squamous cell carcinoma is a type of skin cancer. Squamous cell carcinoma develops from the squamous or basal cells in the epidermis.

Squamous cell carcinoma is a very common type of skin cancer. Most tumours occur in older adults on sun exposed skin usually as a result of the cells in the epidermis being damaged by UV light from the sun.


If the abnormal cells are only seen in the epidermis, the tumour is called squamous cell carcinoma in situ. Once the abnormal cells have broken through the basement membrane to enter the connective tissue and fat below, the diagnosis become squamous cell carcinoma. The movement of cancer cells into the tissue below the epidermis is called invasion.

The diagnosis of squamous cell carcinoma is usually made after a small tissue sample is removed in a procedure called a biopsy. The entire tumour is later removed surgically and sent to a pathologist for examination.

Histologic grade

Pathologists use the word grade to describe the difference between the cancer cells and the normal, healthy squamous cells in the skin.

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

  • Well differentiated - The cancer cells in this tumour look very similar to normal squamous cells.

  • Moderately differentiated - The cancer cells in this tumour are abnormal but they still resemble squamous cells.

  • 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.

Why is this important? 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.

Tumour size

This is the size of the tumour. Tumour size will only be described in your report after the entire tumour has been removed. 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 traveled 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.


Why is this important? Tumours that grow deeper into the dermis are more likely to spread (metastasize) to a lymph node or to grow back after treatment (recurrence).

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.

Why is this important? 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.

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 movement 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.

Why is this important? Lymphovascular invasion increases the risk that cancer cells will be found in a lymph node or a distant part of the body such as the lungs.

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 (metastasize) to a distant site such as the lungs.  


The following microscopic features are considered high risk:

  • A depth of invasion more than 2 millimeters.

  • The presence of cancer cells in the lower half of the skin.

  • Perineural invasion.

  • Lymphovascular invasion.

  • Poorly differentiated tumours.


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 margin is considered positive when there are cancer cells at the very edge of the cut tissue.

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 millimeters.

Why is this important? 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 a metastasis

Lymph nodes are not 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.


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.

Why is this important? Larger tumour deposits are associated with worse prognosis.

Extracapsular extension

All lymph nodes are surrounded by a capsule. Extracapsular extension means that cancer cells have broken through the capsule and into the tissue that surrounds the lymph node.


Why is this important? Extracapsular 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.

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