Actinic keratosis

What is actinic keratosis?

Actinic keratosis (AK) is a pre-cancerous skin disease. It is considered a pre-cancerous disease because, for some patients, the disease will change over time into a type of skin cancer called squamous cell carcinoma. For most patients, however, the disease will persist but will not change into cancer.

actinic keratosis

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.

What causes actinic keratosis?

Actinic keratosis is a very common condition caused by long-term exposure to the sun.  AK commonly occurs on the face, lips, ear, back of the hands, arms and scalp.  However, AK can develop anywhere on the body where the skin has been exposed to the sun for long periods of time.  When similar changes affect the lips, it is called actinic cheilitis.

It is not uncommon for people to have multiple areas of the skin with actinic keratoses. Older age, fair skin, and chronic sun exposure are risk factors for developing actinic keratosis.

How do pathologists make this diagnosis?

When examined without a microscope, AK typically has a scaly red appearance.  The diagnosis is usually made after a small tissue sample is removed in a procedure called a biopsy. The tissue same will then be sent to a pathologist for examination under the microscope.

AK starts in the cells at the very bottom of the epidermis which are called basal cells. When examined under the microscope, the abnormal cells in AK look bigger, darker, and appear more disorganized compared to the more superficial squamous cells.

Types of actinic keratosis

Pathologists divide AK into groups called variants based on how the cells look under the microscope.

The most common variants of AK are:

  • Acantholytic – When examined under the microscope, the cells in this type look like they are separated from each other by a small empty space.
  • Pigmented – This type looks dark when examined under the microscope because the cells contain a pigment called melanin.
  • Hypertrophic – In this type the epidermis is thicker than normal.
  • Bowenoid – In this type, the abnormal cells fill the entire epidermis.

Bowenoid type AK is important because it is associated with a higher risk of changing into a type of cancer called squamous cell carcinoma over time. For this reason, Bowenoid type AK needs to be completely removed in order to reduce the risk of developing cancer and to avoid recurrence.

by Allison Osmond, MD FRCPC (updated June 24, 2021)
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