by Allison Osmond, MD FRCPC
October 22, 2022
Basal cell carcinoma (BCC) is a type of skin cancer. It starts from specialized basal cells that are normally found near the surface of the skin. BCC is the most common human cancer. Most tumours occur in older adults on sun-exposed skin. The cancer cells in BCC rarely spread to lymph nodes or distant sites such as the lungs.
Prolonged and excessive exposure to UV radiation from the sun is the most common cause of BCC. Because they are constantly dividing, basal cells are more sensitive to DNA damage caused by long-term exposure to UV radiation from the sun.
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.
Pathologists divide BCC into histologic types based on how the cancer cells stick together and the shapes they form as the tumour grows. The type of BCC 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 BCC are nodular, superficial, sclerosing, basosquamous, infiltrative, and micronodular. 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 regrow after treatment.
Basal cell carcinoma starts in a layer of tissue at the surface of the skin called the epidermis. The layers below the epidermis are called the dermis and subcutaneous tissue. Pathologists use the term depth of invasion to describe how far the tumour cells have spread from the epidermis into the layers of tissue below (the dermis and subcutaneous tissue). The depth of invasion is important because tumours that grow deeper into the dermis are more likely to regrow after treatment. For skin tumours, 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.
Perineural invasion means that cancer cells were seen attached to a nerve. Nerves are found all over the body and they are responsible for sending information (such as temperature, pressure, and pain) between your body and your brain. Perineural invasion is important because cancer cells that have become attached to a nerve can spread into surrounding tissues by growing along the nerve. This increases the risk that the tumour will regrow after treatment. The infiltrative, sclerosing, and basosquamous types of BCC are more likely to show perineural invasion.
Lymphovascular invasion means that cancer cells were seen inside a blood vessel or lymphatic vessel. Blood vessels are long thin tubes that carry blood around the body. Lymphatic vessels are similar to small blood vessels except that they carry a fluid called lymph instead of blood. Lymphovascular invasion is important because cancer cells can use blood vessels or lymphatic vessels to spread to other parts of the body such as lymph nodes or the lungs. BCC only rarely shows lymphovascular invasion. However, when found it is associated with a higher risk that the cancer cells will spread to lymph nodes.
A margin is a rim of normal tissue that surrounds a tumour and is removed with the tumour at the time of surgery. The margins are usually only described in a pathology report after an excision has been performed to remove the tumour. Margins are often not described after a biopsy.
When examining a BCC 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 regrow in the same site after treatment. The micronodular and infiltrative types of BCC 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.