Basal cell carcinoma is a slow-growing tumor that results from malignant changes in the basal cells of the skin. The body’s largest organ, the skin has a variety of functions including protecting the body from sunlight and heat, controlling body temperature, storing water and fat and synthesizing vitamin D.

The skin has three layers – the epidermis (upper layer), the dermis (middle layer) and the subcutaneous (the deepest layer). Skin cancer develops in the epidermis, which consists of three types of cells:

• Squamous cells. Thin, flat cells that form the top layer of the epidermis.
  
  
• Basal cells. Round cells located under the squamous cells.
  
  
• Melanocytes. Located in the lower part of the epidermis, these cells produce melanin (pigment that gives skin its color and causes skin to darken).

Skin cancer, in general, is the most common type of cancer, possibly accounting for over half of all cancer cases. All forms of skin cancer are divided into two main categories – non-melanoma skin cancer and melanoma skin cancer. Although melanoma skin cancer, which develops in the melanocytes, is much rarer than non-melanoma skin cancer, it is also much more deadly.  

Basal cell carcinoma, which develops in the basal cells, is the most common form of non-melanoma skin cancer. It is also the most common form of cancer in general, accounting for approximately 75 percent of all skin cancers, according to the American Cancer Society (ACS). Although skin cancer appears most often in the basal cells, it can also develop in other cells of the epidermis, such as the squamous cells.

Exposure to the sun is the most common cause of skin cancer. According to the National Institutes of Health (NIH), more than 90 percent of basal cell carcinomas develop on areas of the skin that are regularly exposed to the sun, or other sources of ultraviolet radiation (e.g., tanning beds). It is especially common on the nose and other areas of the head and neck, but can also occur on the back or chest.

The NIH states that basal cell carcinoma most commonly occurs after age 40. Although the disease was typically found in middle-aged or older people in the past, younger people are now being diagnosed with the disease. Many experts believe the occurrence of basal cell carcinoma in younger patients may be due to an increase in sun exposure among this population.

In addition, the disease is approximately twice as common in men as women. This higher occurrence rate is attributed to a greater incidence of sun exposure among men. Patients with basal cell nevus syndrome, a rare congenital (present at birth) condition, are also at an increased risk of developing the disease.

Basal cell carcinoma develops slowly, increasing to only one or two centimeters (0.4 or 0.8 inch) in diameter after several years. It is highly curable and does not usually spread to lymph nodes or distant areas of the body (metastasis) if treated.

If left untreated, however, it can continue to grow and invade neighboring body structures including tissues beneath the skin, bone, nerves and the brain. This may affect the patient’s appearance or the function of the affected body part, such as the nose. This complication is particularly problematic when it occurs around the nose, eyes and ears. Since the disease rarely metastasizes, basal cell carcinoma is not typically staged unless the cancer is very large.

Basal cell cancers can recur in the same place on the skin after treatment. New basal cell cancers can also develop in different areas on the skin. The ACS estimates that 35 to 50 percent of all individuals diagnosed with one basal cell carcinoma will develop a new skin cancer within five years.

According to the NIH, basal cell carcinoma is the most common form of cancer in the United States. According to the ACS, more than 1 million cases of basal cell and squamous cell carcinomas occur each year. The exact number of basal cell carcinoma cases that develop each year is unknown.

In general, the incidence of skin cancer has increased significantly in recent years. Skin cancer is particularly common among lighter-skinned people and uncommon among people with dark skin. The ACS estimates a lifetime risk of developing a basal cell carcinoma at 30 percent for a light-skinned person. Because sun exposure is the greatest cause of the disease, minimizing sun exposure is the best prevention method. States nearer the equator, such as Hawaii and Florida, have much higher skin cancer incidence than more northern states. Protective measures include reducing exposure to other forms of ultraviolet rays, such as those produced in tanning salons. Other risk factors include exposure to arsenic, exposure to radiation and smoking