What Is Skin Cancer and How Does It Develop?

The Basics

Your skin is made up of several layers of cells. Skin cancer develops when some of those cells begin to grow and divide in an uncontrolled way, usually triggered by damage to the cell’s DNA. The most common cause of that damage is ultraviolet (UV) radiation from the sun or tanning beds.

Not all skin cancers are the same. They are named after the type of cell they start in, and that largely determines how they behave and how they are treated.

The Main Types

Other rare skin cancers include dermatofibrosarcoma protuberans (DFSP), sebaceous carcinoma, and angiosarcoma, among others. Surgery still plays an important role in their treatment. Thankfully, these are uncommon as they can be very aggressive — similar to melanoma and Merkel cell carcinoma.

Risk Factors

The most important risk factor for skin cancer is UV exposure — both from the sun over a lifetime and from intense, intermittent burns. Other risk factors include:

• Fair skin, light hair, or light eyes
• A history of sunburns, especially in childhood
• Use of tanning beds
• A weakened immune system (from medications or illness)
• A personal or family history of skin cancer
• Certain inherited conditions (such as xeroderma pigmentosum)
• Chronic skin wounds or scars
• Exposure to radiation or certain chemicals

How Do I Prevent Skin Cancer?

There are meaningful steps you can take to protect your skin:

Report symptoms early — Lumps under the skin, changes to surgical scars, or new skin lesions should be brought to your doctor’s attention rather than watched at home.

Sun protection — Use broad-spectrum SPF 30+ sunscreen daily, wear protective clothing and hats, and seek shade during peak UV hours (10 AM–4 PM).

Avoid tanning beds — There is no safe level of UV exposure from tanning beds.

Perform monthly self-skin exams — Look for new or changing spots, and report anything suspicious to your doctor promptly.

The American Academy of Dermatology recommends use of sunscreen which protects against UV-A and UV-B light, SPF of at least 30, and with water resistance.

Helpful Links and More Resources

• aad.org — Sun Protection
• aad.org — Sunscreen Statistics
• aad.org — Indoor Tanning Statistics

Diagnosis and Biopsy

The Basics

Skin cancer is diagnosed by taking a small sample of the suspicious area — called a biopsy — and examining it under a microscope. This is a minor procedure done in a clinic or office, usually with just a local anesthetic (numbing medicine).

A biopsy is the only way to know for certain whether a lesion is cancerous and, if so, what type it is. The results typically take several days up to two weeks.

Types of Biopsy

The type of biopsy your doctor recommends depends on the size, location, and appearance of the lesion:

Shave biopsy — A thin layer of the lesion is shaved off using a small blade. This is commonly used for raised or superficial lesions. It is quick and leaves a minimal scar.

Punch biopsy — A small circular tool is used to remove a cylindrical core of skin, including deeper layers. This is often preferred when the full thickness of the skin needs to be evaluated, such as for melanoma.

Excisional biopsy — The entire lesion is surgically removed along with a small border of normal-appearing skin. This is sometimes done when the lesion is small enough to remove completely at the time of biopsy.

Incisional biopsy — Only a portion of a larger lesion is removed. This may be done when a full excision is not practical at the time of the initial evaluation. This is uncommonly performed given the ease and precision of punch biopsy.

Reading Your Pathology Report

After your biopsy, a pathologist (a doctor who specializes in examining tissue under a microscope) will analyze the sample and write a report. This report is the foundation of your diagnosis. Key things it typically includes:

• Diagnosis — the type of skin cancer (or whether the lesion is benign/non-cancer)
• Breslow thickness (for melanoma) — how deep the melanoma extends into the skin, measured in millimeters; this is one of the most important factors in determining treatment and prognosis
• Ulceration (for melanoma) — whether the surface of the melanoma has broken down, which affects staging
• Margins — whether cancer cells are present at the edges of the removed tissue
• Mitotic rate — how rapidly the cancer cells are dividing
• Perineural or lymphovascular invasion — whether cancer cells have grown into nearby nerves or blood vessels, which can affect the risk of spread

It is completely normal to have questions about your pathology report. Your surgeon or dermatologist will walk you through what it means for your specific situation.

Staging and Prognosis

The Basics

Staging is the process of determining how far a cancer has spread. It helps your medical team choose the right treatment and gives you and your family a sense of what to expect. Staging uses information from your biopsy, imaging studies, and — for some cancers — the sentinel lymph node biopsy.

Most cancers in the United States are staged using the TNM system, developed by the American Joint Committee on Cancer (AJCC):

Melanoma Staging

Melanoma staging is among the most detailed of any cancer because even small differences in tumor thickness can meaningfully affect prognosis.

Stage I and II — Melanoma confined to the skin. Stage is determined primarily by Breslow thickness and whether ulceration is present. Most patients with Stage I or II melanoma are cured with surgery alone, though higher-risk Stage II patients may benefit from adjuvant immunotherapy.

Stage III — Melanoma has spread to regional lymph nodes or nearby skin (satellite or in-transit metastases). Treatment typically involves surgery to remove the affected lymph nodes, followed by adjuvant immunotherapy or targeted therapy to reduce the risk of recurrence.

Stage IV — Melanoma has spread to distant organs such as the lungs, liver, brain, or bone. Treatment options have improved dramatically with modern immunotherapy and targeted therapy, and some patients achieve long-term remission. However, Stage IV melanoma remains a serious diagnosis that requires close management by a multidisciplinary team.

Basal Cell and Squamous Cell Carcinoma Staging

BCC and SCC are also staged using the TNM system, though staging is less commonly emphasized in clinical practice because the vast majority of these cancers are caught early and treated successfully with surgery. Staging becomes more relevant for high-risk or locally advanced tumors and those that have spread to lymph nodes.

Merkel Cell Carcinoma Staging

MCC is staged similarly to other skin cancers, but because it is more aggressive, staging plays a critical role in treatment planning from the outset. Even Stage I MCC typically warrants sentinel lymph node biopsy and consideration of adjuvant radiation.

A Word on Prognosis

Prognosis — the expected outcome — is different for every patient. The numbers your doctor may share with you (such as 5-year survival rates) are based on large groups of patients and may not reflect your individual situation. Factors like overall health, tumor biology, treatment response, and advances in therapy all matter. If you have questions about what your specific diagnosis means for your outlook, members of your oncology team are the best people to talk to.

Surgical Treatment

The Basics

Surgery is the primary treatment for most skin cancers. The goal is to remove all of the cancer with a margin of normal tissue around it to reduce the chance of it coming back. The type and extent of surgery depend on the type of skin cancer, its size, its location, and how deep it has grown.

Wide Local Excision

The most common surgical approach is a wide local excision — removing the tumor along with a measured border of surrounding normal skin called a surgical margin. BCC and SCC can also be treated with other modalities by your dermatologist, including Mohs surgery (see below). Typically, formal surgical referral for these cancers is reserved for when the lesion is large or appears to have spread. The width of that margin is guided by the type of cancer and national surgical guidelines:

After excision, the wound is usually closed with sutures. Depending on the size and location, a simple side-to-side closure, a skin flap, or a skin graft may be needed.

Mohs Micrographic Surgery

Mohs surgery is a specialized technique performed by dermatologists trained in this method. It involves removing the tumor one thin layer at a time and immediately examining each layer under a microscope before proceeding. This continues until no cancer cells remain at the edges.

Mohs offers the highest cure rates for certain skin cancers while removing as little normal tissue as possible. It is particularly useful for:

• Tumors on the face, ears, nose, lips, or eyelids — where preserving tissue is important
• Tumors with ill-defined borders
• Recurrent skin cancers
• Tumors in high-risk locations or with aggressive features

Mohs surgery is primarily used for BCC and SCC. For melanoma and Merkel cell carcinoma, standard wide local excision with predetermined margins is more commonly used, though Mohs-based techniques are being studied.

When Surgery Is Not Straightforward

Some skin cancers require more complex planning — for example, when the tumor is large, located near critical structures, has recurred after prior treatment, or when reconstruction is needed after removal. In these cases, a multidisciplinary team including surgical oncology, plastic surgery, medical oncology, and radiation oncology may be involved in your care.

Sentinel Lymph Node Biopsy

The Basics

For certain skin cancers — most commonly melanoma and Merkel cell carcinoma — there is a risk that cancer cells may have traveled to the nearest lymph nodes before the diagnosis was made. A sentinel lymph node biopsy (SLNB) is a minimally invasive procedure that checks whether this has happened, without removing all of the lymph nodes in the region.

The sentinel lymph node is the first lymph node (or nodes) that drains fluid from the area where the tumor is located. If cancer has spread to the lymph nodes, it almost always reaches the sentinel node first. Checking it gives important information about the stage of your cancer and helps guide further treatment decisions.

Who Needs a Sentinel Lymph Node Biopsy?

Melanoma: SLNB is generally recommended for melanomas that are 0.8 mm or thicker, or thinner melanomas with certain high-risk features (such as ulceration or a high mitotic rate). It is typically not recommended for melanomas thinner than 0.8 mm without these features, as the risk of lymph node spread is very low.

Merkel cell carcinoma: SLNB is recommended for most patients with MCC, as it has a relatively high rate of spreading to regional lymph nodes even when the primary tumor is small.

Squamous cell carcinoma: SLNB may be considered in select high-risk SCC cases, though it is not routinely performed.

How the Procedure Works

SLNB is typically done at the time of the wide local excision, under general or regional anesthesia, as a same-day or short-stay procedure.

1. A small amount of a radioactive tracer and/or a blue dye is injected near the tumor site.
2. The tracer travels through the lymphatic channels to the sentinel node(s), which are identified using a handheld detector or visual inspection for blue dye.
3. The sentinel node(s) are removed and sent to a pathologist for examination.
4. Results are usually available within a few days to a week.

If the sentinel lymph node is negative (no cancer found), no further lymph node surgery is typically needed. If it is positive (cancer found), your care team will discuss next steps, which may include watchful waiting, or in some circumstances, additional surgery, radiation, or systemic therapy.

Understanding Your Results

A negative SLNB means that no cancer cells were found in the sentinel node. This is a reassuring finding and means the cancer is less likely to have spread further. It does not guarantee that no cancer cells exist elsewhere, but it significantly reduces that likelihood.

A positive SLNB means that cancer cells were found. This affects your staging and may change your treatment plan. For melanoma, a positive sentinel node changes the stage to Stage III, and additional treatments such as immunotherapy are often recommended.

Systemic Therapy

The Basics

Systemic therapy refers to treatments that work throughout the entire body — as opposed to surgery or radiation, which target a specific area. For skin cancer, systemic therapy is used when the cancer has spread beyond the skin to lymph nodes or distant organs, or in certain high-risk situations to reduce the chance of recurrence after surgery.

The most important systemic therapies for skin cancer today are immunotherapy and targeted therapy. Chemotherapy, which is relatively ineffective for melanoma, is now rarely used except in certain cases of squamous cell cancer and more rare skin cancers.

Immunotherapy

Immunotherapy works by helping your own immune system recognize and attack cancer cells. Healthy immune systems have natural “brakes” that prevent them from attacking normal tissue — but cancer cells can exploit these brakes to hide from detection. Immunotherapy drugs called checkpoint inhibitors release these brakes, allowing the immune system to fight the cancer more effectively.

For melanoma, checkpoint inhibitors have transformed treatment over the past decade. The most commonly used agents target proteins called PD-1 (pembrolizumab, nivolumab) or CTLA-4 (ipilimumab), or a combination of both. These drugs are used:

• After surgery in patients at high risk of recurrence (adjuvant therapy)
• Before surgery in select cases to shrink the tumor (neoadjuvant therapy)
• As the primary treatment for metastatic melanoma

For Merkel cell carcinoma, checkpoint inhibitors (avelumab, pembrolizumab) are now a first-line treatment for advanced disease and are showing promise in earlier stages.

For squamous cell carcinoma, cemiplimab and pembrolizumab are approved for locally advanced or metastatic SCC that cannot be treated with surgery or radiation.

Side effects of immunotherapy can vary widely. Many patients tolerate it well, but because it activates the immune system broadly, it can sometimes cause inflammation in normal organs — a category of side effects called immune-related adverse events (irAEs). Common ones include skin rashes, diarrhea, and thyroid problems. Serious but less common effects can involve the lungs, liver, or other organs. Your oncology team will monitor you closely during treatment.

Targeted Therapy

Some cancers have specific genetic mutations that drive their growth. Targeted therapy uses drugs designed to block those specific mutations.

For melanoma, approximately 40–60% of cases have a mutation in the BRAF gene (most commonly BRAF V600E). Patients with this mutation can be treated with a combination of BRAF and MEK inhibitors (such as dabrafenib + trametinib, or vemurafenib + cobimetinib). These drugs can produce rapid and significant responses, though resistance can develop over time.

Genetic testing of your tumor (molecular profiling) is now a standard part of the workup for advanced melanoma to determine whether targeted therapy is an option.

For basal cell carcinoma, a small number of patients with locally advanced or metastatic BCC that cannot be treated with surgery or radiation may be candidates for hedgehog pathway inhibitors (vismodegib or sonidegib), which target a signaling pathway that is abnormally activated in most BCCs.

A Note on Who Prescribes Systemic Therapy

Systemic therapy for skin cancer is typically managed by a medical oncologist, often in collaboration with the surgical team. Surgical oncology works closely with medical oncologists and will help coordinate your care and ensure the right specialists are involved.

Survivorship and Surveillance

The Basics

Completing treatment for skin cancer is a significant milestone — but it is not the end of your care. Survivorship focuses on your health and wellbeing after treatment, and surveillance means regular follow-up appointments and skin checks to catch any recurrence or new skin cancers as early as possible.

Skin Surveillance

Skin cancer survivors have a higher-than-average risk of developing a new skin cancer in the future. Regular full-body skin exams — performed by a dermatologist — are an important part of survivorship care. How often you need these exams depends on the type of cancer you had, your risk factors, and whether you have had multiple skin cancers.

In general:

• Patients with a history of BCC or SCC should have a full-body skin exam at least once a year, often every 6 months for the first few years.
• Melanoma survivors typically have more frequent skin and lymph node checks, especially in the first 2–5 years after treatment.
• MCC survivors require close surveillance given the higher risk of recurrence.

Imaging and Lab Tests

For patients with higher-stage cancers — particularly Stage III or IV melanoma or MCC — periodic imaging (such as CT scans or PET scans) may be part of the surveillance plan to look for signs of recurrence in lymph nodes or distant organs. Your care team will discuss how often imaging is recommended based on your specific situation.

Reducing Your Risk of Recurrence and New Skin Cancers

There are meaningful steps you can take to protect your skin and reduce your risk going forward:

• Sun protection — Use broad-spectrum SPF 30+ sunscreen daily, wear protective clothing and hats, and seek shade during peak UV hours (10 AM–4 PM).
• Avoid tanning beds — There is no safe level of UV exposure from tanning beds.
• Perform monthly self-skin exams — Look for new or changing spots, and report anything suspicious to your doctor promptly.
• Report symptoms early — Lumps under the skin, changes to surgical scars, or new skin lesions should be brought to your doctor’s attention rather than watched at home.

Emotional and Psychological Wellbeing

A cancer diagnosis — even one with an excellent prognosis — can take a significant emotional toll. Anxiety about recurrence, changes in body image after surgery, and the psychological weight of ongoing surveillance are all real and common experiences. You do not need to navigate these alone.

Many patients find it helpful to speak with a counselor or therapist experienced in working with cancer patients. Support groups — both in person and online — can also provide connection with others who understand what you are going through. If you are struggling emotionally, please let your care team know. Addressing your mental and emotional health is a legitimate and important part of cancer survivorship.

Long-Term Side Effects of Treatment

Depending on the treatment you received, you may experience long-term or late effects:

• Surgery can cause scarring, numbness, or lymphedema (swelling from lymph node removal) in the affected area.
• Radiation can cause skin changes, fatigue, or fibrosis in the treated area over time.
• Immunotherapy can occasionally cause lasting effects on the thyroid, adrenal glands, or other organs even after treatment ends.

Your care team will monitor for these effects and help manage them if they occur.