About the skin
The skin is the body’s largest organ. It protects the body against infection and injury and helps regulate body temperature. The skin also stores water and fat and produces vitamin D.
The skin is made up of 3 main layers:
- The epidermis. The outer layer of skin.
- The dermis. The inner layer of skin.
- The hypodermis. The deep layer of fat.
See the Medical Illustrations section for a drawing of these layers.
About skin cancer
Cancer begins when healthy cells change and grow out of control, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but will not spread.
Doctors diagnose skin cancer in more than 3 million Americans each year, making it the most common type of cancer. If skin cancer is found early, it can usually be treated with topical medications, procedures done in the office by a dermatologist, or an outpatient surgery. A dermatologist is a doctor who specializes in diseases and conditions of the skin. As a result, skin cancer is responsible for less than 1% of all cancer deaths.
In some cases, skin cancer may be more advanced and require management by a multidisciplinary team that often includes a dermatologist, surgical oncologist, radiation oncologist, and a medical oncologist. These doctors will meet with a patient, and together they will recommend the best path forward to treat the cancer. In some instances, the surgical oncologist will recommend a surgery to be performed in an operating room when the procedure to treat the cancer is too extensive for an office setting. Other times, radiation therapy and/or treatments using medication given by mouth or by vein are recommended by the team either in place of or in combination with surgery. (More details are given in the Types of Treatment section of this guide.)
Types of skin cancer
There are 4 main types of skin cancer:
- Basal cell carcinoma. Basal cells are the round cells found in the lower epidermis. About 80% of skin cancers develop from this type of cell. These cancers are described as basal cell carcinomas. Basal cell carcinoma most often develops on the head and neck, although it can be found anywhere on the skin. It is mainly caused by sun exposure or develops in people who received radiation therapy as children. This type of skin cancer usually grows slowly and rarely spreads to other parts of the body.
- Squamous cell carcinoma. Most of the epidermis is made up of flat, scale-like cells called squamous cells. Around 20% of skin cancers develop from these cells, and these cancers are called squamous cell carcinomas. Squamous cell carcinoma is mainly caused by sun exposure, so it may be diagnosed on many regions of the skin. It can also develop on skin that has been burned, damaged by chemicals, or exposed to x-rays. Squamous cell carcinoma is commonly found on the lips; at sites of a long-standing scar; and on the skin outside the mouth, anus, and vagina. About 2% to 5% of squamous cell carcinomas spread to other parts of the body.
- Merkel cell cancer. Merkel cell cancer is a highly aggressive, or fast-growing, rare cancer. It starts in hormone-producing cells just beneath the skin and in the hair follicles. It is usually found in the head and neck region. Merkel cell cancer may also be called neuroendocrine carcinoma of the skin. Learn more about neuroendocrine tumors.
- Melanoma. There are scattered cells called melanocytes where the epidermis meets the dermis. These cells produce the pigment melanin, which gives skin its color. Melanoma starts in melanocytes, and it is the most aggressive type of skin cancer. It accounts for about 1% of all skin cancers. For more information about melanoma, visit the melanoma section on this same website.
Basal cell carcinoma and squamous cell carcinoma are sometimes grouped together and called keratinocyte carcinoma. This is because they begin in a type of skin cell called a keratinocyte. In this guide, “non-melanoma skin cancer” refers to keratinocyte carcinoma and Merkel cell cancer to distinguish them from melanoma.