Melanoma is a cancer that starts in the cells that make skin pigment

Melanoma begins in melanocytes—the cells in your skin that produce melanin, the pigment that gives skin its color. Unlike other skin cancers that grow slowly, melanoma can spread to other parts of your body if not caught early. It accounts for a smaller share of skin cancer diagnoses than basal cell or squamous cell cancers, but it is responsible for most skin cancer deaths because of its tendency to spread.

The disease develops when melanocytes start to grow abnormally. This can happen on skin that gets regular sun exposure, but melanoma also appears on areas that rarely see the sun. Some people develop melanoma from an existing mole; others develop it in what looked like normal skin. The cancer can appear as a new dark spot or as a change in an existing mole.

Key Takeaways

  • Melanoma starts in melanocytes, the pigment-producing cells in your skin, and can spread to lymph nodes and other organs if not treated early.
  • The ABCDE rule—asymmetry, border irregularity, color variation, diameter larger than a pencil eraser, and evolving or changing—helps identify suspicious moles worth showing a doctor.
  • Risk factors include fair skin, family history of melanoma, severe sunburns in childhood, and having many moles or atypical moles.
  • Melanoma is staged based on how deep it goes into the skin and whether it has spread, which determines treatment options and prognosis.
  • Early detection through regular skin checks and prompt removal of suspicious lesions significantly improves outcomes.

How melanoma develops and spreads

Melanoma begins when DNA damage in melanocytes causes them to divide uncontrollably. This damage is often triggered by ultraviolet (UV) radiation from the sun or tanning beds, though genetics and other factors also play a role. The cancer cells form a tumor in the outer layer of skin called the epidermis.

If melanoma is not removed, it can grow downward into deeper layers of skin. Once it reaches the dermis—the layer beneath the epidermis—it has a higher chance of spreading to lymph nodes and distant organs like the lungs, liver, or brain. The depth of the tumor and whether cancer cells have reached blood vessels or lymph vessels are key factors in determining how likely the cancer is to spread.

The ABCDE warning signs

Doctors use the ABCDE rule to help people recognize melanoma. Asymmetry means one half of a mole does not match the other. Border irregularityColor variationDiameterEvolving

Any mole or spot that fits one or more of these criteria warrants a visit to a dermatologist. Changes that happen quickly—over days or weeks rather than years—are especially worth reporting. Some melanomas do not follow the ABCDE pattern, so any new or changing skin growth should be evaluated by a doctor.

Who is at higher risk

People with fair skin, light hair, and light eyes have a higher risk of melanoma than those with darker skin, though melanoma does occur across all skin types. A family history of melanoma increases risk significantly—if a parent or sibling had melanoma, your risk is higher. Having many moles, especially atypical moles (also called dysplastic nevi) that are irregular in shape or color, also raises risk.

Severe sunburns in childhood, particularly blistering burns, increase melanoma risk in adulthood. Repeated sun exposure over a lifetime matters too, as does use of tanning beds. A personal history of melanoma means there is a risk of developing another melanoma later. People with weakened immune systems face higher risk as well.

How melanoma is staged

Once melanoma is diagnosed, doctors stage it to understand how far it has progressed. Stage 0 (melanoma in situ) means cancer cells are only in the epidermis and have not invaded deeper skin. Stage 1 melanomas are localized to the skin and are less than 2 millimeters thick. Stage 2 melanomas are thicker or have other high-risk features but have not spread to lymph nodes. Stage 3 means cancer has spread to nearby lymph nodes or skin. Stage 4 indicates the cancer has spread to distant organs.

The thickness of the tumor, measured in millimeters, is one of the most important predictors of outcome. Melanomas caught when they are thin (less than 1 millimeter) have much higher survival rates than thicker tumors. Whether the tumor has ulceration (a break in the skin surface) and how many mitotic figures (dividing cells) are present also affect staging and treatment decisions.

Diagnosis and biopsy

If a dermatologist suspects melanoma based on appearance, a skin biopsy is needed to confirm the diagnosis. During a biopsy, the doctor removes all or part of the suspicious lesion and sends it to a pathologist, who examines the cells under a microscope. The pathologist looks for melanoma cells and measures the thickness, depth, and other features that determine stage and guide treatment.

If melanoma is confirmed, additional imaging tests may be ordered depending on the stage. These can include ultrasound of nearby lymph nodes, CT scans, or PET scans to check whether cancer has spread. Not all melanomas require these tests—early-stage melanomas that have not spread may only need the biopsy result and careful follow-up.

Treatment options depend on stage and depth

Early-stage melanomas are typically treated with surgical removal. The surgeon removes the melanoma along with a margin of normal skin around it—the margin size depends on how thick the tumor is. For very thin melanomas, a small margin may be sufficient. Thicker melanomas require a wider margin to reduce the chance that cancer cells were left behind.

If melanoma has spread to lymph nodes, the affected nodes may be removed surgically. For advanced melanomas that have spread to distant organs, treatment may include immunotherapy drugs, targeted therapy drugs that work against specific mutations, chemotherapy, or radiation. The specific treatment depends on the stage, the location of spread, and genetic testing of the tumor cells.

Frequently Asked Questions

Can melanoma develop from a mole I have had my whole life?

Yes, melanoma can develop from an existing mole, though most moles never become cancer. If a mole you have had for years suddenly starts changing in size, shape, or color, that change warrants a doctor's evaluation. Moles that have been stable for decades are less likely to become melanoma than new moles or ones that are actively changing.

Is all melanoma caused by sun exposure?

Sun exposure is a major risk factor, but not all melanoma is caused by the sun. Melanoma can develop on skin that rarely sees sunlight, such as the soles of feet, palms, or under nails. Genetics, family history, and the number of moles you have also influence risk. People with darker skin can develop melanoma even with minimal sun exposure.

What is the difference between melanoma and other skin cancers?

Basal cell and squamous cell cancers grow slowly and rarely spread to other parts of the body, even if left untreated for years. Melanoma, though less common, spreads much more readily to lymph nodes and distant organs. This is why melanoma is more dangerous and why early detection and removal are critical.

If melanoma is caught early, what is the survival rate?

Melanomas caught when they are thin and localized to the skin have five-year survival rates above 90 percent. Once melanoma has spread to lymph nodes or distant organs, survival rates drop significantly. This is why regular skin checks and prompt removal of suspicious lesions make such a difference in outcomes.

Do I need to check my skin myself, or is a doctor visit enough?

Both matter. A dermatologist can spot changes you might miss, but you see your skin daily and notice new spots or changes fastest. Monthly self-checks using the ABCDE rule help catch changes early. People at higher risk should see a dermatologist annually or more often; those at average risk may need less frequent visits.