Basal cell carcinoma of the skin

What is basal cell carcinoma?

Basal cell carcinoma is a type of skin cancer. It starts from specialized basal cells that are normally found near the surface of the skin. Basal cell carcinoma is the most common human cancer. Most tumours occur in older adults on sun-exposed skin. The tumour cells in basal cell carcinoma rarely spread to lymph nodes or distant sites such as the lungs.

basal cell carcinoma

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 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.

Basal cells are normally found at the very bottom of the epidermis, just above the dermis (see picture below). As the squamous cells in the skin die and fall off the body, the basal cells divide to create new squamous cells. Because they are constantly dividing, basal cells are more sensitive to DNA damage caused by long-term exposure to UV radiation from the sun.

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.

Types of basal cell carcinoma

There are different types of basal cell carcinoma based on how the cancer cells stick together and the shapes they form as the tumour grows. The type of basal cell carcinoma can only be determined after the tumour has been examined under a microscope by a pathologist.  A tumour may be made up of one or multiple types of basal cell carcinoma.

The most common types of basal cell carcinoma are:

  • Nodular
  • Superficial
  • Multifocal
  • Sclerosing
  • Basosquamous
  • Infiltrative
  • Micronodular

basal cell carcinoma

The sclerosing, basosquamous, and infiltrative types tend to grow deeper into the tissue below the epidermis and are more difficult to remove completely. As a result, they are associated with a higher risk that the tumour will re-grow after treatment.

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

Tumour size

This is the size of the tumour measured in centimetres. Your report may only describe the greatest dimension. For example, if the tumour measures 5.0 cm by 3.2 cm by 1.1 cm, the report may describe the tumour size as 5.0 cm in the greatest dimension. The tumour size is only described after the entire tumour has been removed. Tumour size is not reported after a biopsy. The tumour size is important because it is used to determine the tumour stage (see Pathologic stage below) and because tumours that are larger than 2 centimetres are more likely to re-grow after treatment.

Depth of invasion

All basal 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 invade deep into the dermis or subcutaneous adipose tissue are more likely to re-grow after treatment. The depth of invasion is also used to determine the tumour stage (see pathologic stage below). Tumours that grow into bone are associated with a worse prognosis.

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. Tumours with basosquamous and sclerosing patterns of growth are more likely to show perineural invasion.

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 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. Basal cell carcinoma only rarely shows lymphovascular invasion however when present it is associated with a higher risk that the cancer cells will spread to lymph nodes.

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 location or to spread to a distant site such as the lungs. Cancer cells that spread to other parts of the body are called metastasis.

High-risk features for basal cell carcinoma include:

  • Depth of invasion more than 2 millimetres
  • Invasion into the lower half of the skin
  • Perineural invasion (see section above)
  • Lymphovascular invasion (see section above)
  • Tumours that involve the ear or hair-bearing lip
Margins

A margin is a rim of normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. Margins will only be described in your report after the entire tumour has been removed. Margins are not described after a biopsy.

When examining a basal cell carcinoma under the microscope, a margin is considered positive when there is no distance between the cancer cells and the cut edge of the tissue. A margin is called negative when there are no cancer cells at the cut edge of the tissue.

A positive margin is associated with a higher risk that the tumour will re-grow in the same site after treatment. The micronodular and infiltrative patterns (see Patterns of growth above) are associated with a higher risk of a positive margin because there is no clear boundary between the edge of the tumour and the adjacent normal tissue.

Margin

by Allison Osmond, MD FRCPC (updated September 8, 2021)
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