Basal cell carcinoma
This article was last reviewed and updated on November 1, 2019
by Allison Osmond, MD FRCPC
Basal cell carcinoma is a type of skin cancer.
It is the most common type of cancer in adults.
Basal cell carcinoma affects older adults and it starts in sun exposed 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 basal cell carcinoma?
Basal cell carcinoma is a type of skin cancer. Basal cell carcinoma starts from the basal cells in the epidermis at 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.
The diagnosis of basal cell carcinoma is usually made after a small sample of tissue is removed in a procedure called a biopsy. Surgery is then performed to remove the entire tumour.
Pattern of growth
Pattern of growth describes the way the cancer cells stick together and the shapes they form as the tumour grows. Basal cell carcinoma can have several patterns of growth when viewed under the microscope. Your tumor may have one pattern of growth or a combination of patterns.
Patterns of growth include:
Why is this important? The sclerosing, basosquamous, and infiltrative patterns of growth are associated with a higher risk that the tumor will re-grow after treatment.
This is the size of the tumour measured in centimeters. 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 greatest dimension.
The tumour size is only described after the entire tumour has been removed. Tumour size is not reported after a biopsy.
Why is this important? The tumour size is used to determine the tumour stage (see Pathologic stage below). Tumours that are larger than 2 centimeters 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 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 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 worse prognosis.
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.
Tumours with basosquamous and sclerosing patterns of growth are more likely to show perineural invasion.
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.
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? 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.
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 a metastasis.
High risk features for basal cell carcinoma include:
Depth of invasion more than 2 millimeters.
Invasion into the lower half of the skin.
Tumours that involve the ear or hair bearing lip.
Please see the sections above for more information on each of these high risk features.
A margin is a rim of normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. 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.
Margins will only be described in your report after the entire tumour has been removed. Margins are not described after a biopsy.
Why is this important? 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.
Pathologic stage (pTNM)
The pathologic stage for basal cell carcinoma is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer.
This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M) to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and worse prognosis.
Tumour stage (pT) for basal cell carcinoma
Basal cell carcinoma is assigned a tumour stage between 1 and 4 based on the size of the tumour, the depth of invasion, and the presence of high-risk features (see high-risk features above). Specifically, tumours that are greater than 2 cm or have invaded into bone are assigned a high stage.
Nodal stage (pN) for basal cell carcinoma
Basal cell carcinoma is assigned a nodal stage between 0 and 3 based on the presence of tumour within a lymph node, the size of the group of cancer cells in the lymph node, the number of lymph nodes involved, and whether the involved lymph nodes are on the same or opposite side of the body as the main tumour.
If no lymph nodes are involved the nodal stage is N0. If no lymph nodes are submitted for pathological examination, the N stage cannot be determined and the nodal stage is listed as NX.
Metastatic stage (pM) for basal cell carcinoma
Basal cell carcinoma is given an metastatic stage between 0 and 1 based on the presence of cancer cells at a distant site in the body (for example the lungs). The metastatic stage can only be determined if tissue from a distant site is submitted for pathological examination. Because this tissue is rarely present, the metastatic stage cannot be determined and is listed as MX.