Skin Cancer

Skin Cancer: Types, Causes, Symptoms, and What You Need to Know

Skin cancer is the most commonly diagnosed cancer in the United States and worldwide. It develops when abnormal cells in the skin grow uncontrollably, typically as a result of DNA damage — most often caused by ultraviolet (UV) radiation from the sun or tanning devices.

Unlike many other cancers, the majority of skin cancers are visible to the naked eye, which gives them a significant advantage in early detection and treatment. However, not all skin cancers are created equal: some are slow-growing and rarely spread, while others are aggressive and potentially life-threatening.

Understanding the different types of skin cancer, what distinguishes them from one another and from other cancers, how they originate, what warning signs to look for, and where they may spread is essential knowledge — both for prevention and for catching the disease early, when it is most treatable.

What Makes Skin Cancer Different from Other Cancers

All cancers share a fundamental characteristic: normal cells undergo mutations that cause them to divide uncontrollably, invade surrounding tissue, and potentially spread to other parts of the body. Skin cancer follows this same biological pattern. What distinguishes it from cancers of internal organs — such as lung, breast, or colon cancer — is primarily the organ of origin, the causative factors, and, in most cases, the ability to detect it early through direct visual inspection.

Several features set skin cancer apart:

• Visibility. Because skin is the body’s outermost layer, tumors often appear as visible changes — growths, discolorations, sores, or lesions — long before they cause systemic symptoms. This contrasts sharply with internal cancers, which can grow silently for years.
• Primary cause. While most cancers have multifactorial origins (genetics, environment, lifestyle), the overwhelming majority of skin cancers are directly attributable to UV radiation exposure. This makes skin cancer among the most preventable of all cancers.
• Cell of origin. Skin cancers arise from the specific cell types that make up the epidermis and deeper skin layers, including keratinocytes, melanocytes, Merkel cells, and cells of the skin’s adnexal structures (hair follicles, sweat glands).
• Treatment response. Non-melanoma skin cancers (basal cell and squamous cell carcinoma) are typically localized, highly amenable to surgical removal, and rarely fatal. Melanoma and Merkel cell carcinoma, by contrast, can metastasize early and require systemic treatment.
• Prognosis spectrum. The prognosis for skin cancer ranges from nearly 100% cure rates (basal cell carcinoma caught early) to substantial mortality risk (metastatic melanoma or Merkel cell carcinoma), depending on type and stage.

How Skin Cancer Starts

The skin is composed of multiple layers. The outermost layer, the epidermis, contains several types of cells. Keratinocytes make up the vast majority of the epidermis and are responsible for forming the skin’s protective barrier. Melanocytes, found at the base of the epidermis (the stratum basale), produce melanin, the pigment that gives skin its color and absorbs UV radiation. Merkel cells, also located in the basal layer, are involved in the sensation of light touch.

When UV radiation penetrates the skin, it can cause direct damage to the DNA within these cells — particularly mutations in tumor suppressor genes such as TP53 (in keratinocytes) and CDKN2A, or oncogene activations such as BRAF mutations (commonly found in melanoma). Normally, the body either repairs this DNA damage or triggers the damaged cell to undergo apoptosis (programmed cell death). When these protective mechanisms fail — often as a result of repeated, cumulative UV exposure over years — a mutated cell may begin dividing uncontrollably, giving rise to a cancerous lesion.

In some cases, skin cancer does not originate from UV damage. Genetic predisposition, chronic inflammation, immunosuppression (such as in organ transplant recipients), exposure to certain chemicals (arsenic, coal tar), or infection with human papillomavirus (HPV) can also contribute to malignant transformation.

Skin cancer typically begins as a localized, in situ lesion — meaning the abnormal cells are confined to the epidermis and have not yet invaded deeper layers. Over time, without detection or treatment, many cancers progress through deeper layers of the dermis and subcutaneous tissue, eventually reaching lymphatic vessels and blood vessels, which provide pathways for metastatic spread.

Types of Skin Cancer

Skin cancers are broadly categorized as non-melanoma skin cancers (NMSC) and melanoma. Non-melanoma skin cancers are further divided into basal cell carcinoma and squamous cell carcinoma, which together account for the overwhelming majority of cases. However, the skin can give rise to a diverse range of malignant tumors. Below is a comprehensive overview of all recognized types.

1. Basal Cell Carcinoma (BCC)

Basal cell carcinoma is the most common cancer in humans, accounting for approximately 80% of all skin cancers. It arises from basal cells — the round cells found in the deepest layer of the epidermis. BCC grows very slowly, almost never spreads (metastasizes) to other organs, and is rarely life-threatening when caught early. However, if neglected, it can invade deeply into surrounding tissue, muscle, and bone, causing significant local destruction.

BCC is strongly associated with chronic sun exposure and typically appears on sun-exposed areas: the face, nose, ears, neck, scalp, and hands. It can present in several subtypes, including nodular (the most common), superficial, morpheaform (sclerosing), pigmented, and basosquamous (which has features of both BCC and SCC and carries a higher risk of metastasis).

2. Squamous Cell Carcinoma (SCC)

Squamous cell carcinoma is the second most common skin cancer, arising from squamous cells — the flat cells that form the outer layers of the epidermis. SCC has a higher metastatic potential than BCC, particularly when it arises on the lips, ears, or inside the mouth, or when it develops in individuals who are immunosuppressed. Overall, however, most SCCs are caught and treated before spreading.

SCC can develop from precancerous lesions called actinic keratoses (rough, scaly patches caused by years of UV exposure) or from Bowen’s disease (SCC in situ, meaning it has not yet invaded the dermis). It may also arise in chronic wounds, scars, or areas of chronic inflammation — a phenomenon known as a Marjolin’s ulcer. Risk factors include UV exposure, fair skin, immunosuppression, HPV infection, arsenic exposure, and tobacco use (particularly for oral SCC).

3. Melanoma

Melanoma arises from melanocytes, the pigment-producing cells of the skin. While it accounts for only about 1–5% of skin cancers by incidence, melanoma is responsible for the vast majority of skin cancer deaths due to its strong tendency to metastasize early. The global incidence of melanoma has been rising for decades.

Melanoma may arise in an existing mole (nevus) or appear as a new lesion on otherwise normal-looking skin. It can occur anywhere on the body, including areas not typically exposed to the sun. There are four main clinical and histological subtypes:

• Superficial spreading melanoma — the most common type, growing horizontally across the skin surface before invading deeper layers.
• Nodular melanoma — a more aggressive form that grows vertically into the skin from the outset, often presenting as a rapidly growing, darkly pigmented, dome-shaped lesion.
• Lentigo maligna melanoma — develops within a lentigo maligna (a flat, pigmented precursor lesion), typically on the chronically sun-damaged skin of older individuals.
• Acral lentiginous melanoma — occurs on the palms, soles, and under the nails (subungual); it is the most common melanoma subtype in people with darker skin tones and is not associated with UV exposure.

Additional rare subtypes include desmoplastic melanoma, amelanotic melanoma (lacking pigment, making it particularly easy to overlook), mucosal melanoma, and uveal (ocular) melanoma.

4. Merkel Cell Carcinoma (MCC)

Merkel cell carcinoma is a rare but aggressive form of skin cancer arising from Merkel cells, which are mechanoreceptors located in the basal layer of the epidermis. MCC is associated with infection by Merkel cell polyomavirus (MCPyV), UV exposure, and immunosuppression. It typically appears as a flesh-colored or bluish-red nodule, most often on the face, head, or neck. MCC has a high rate of local recurrence and distant metastasis, resulting in a significantly worse prognosis than most non-melanoma skin cancers.

5. Dermatofibrosarcoma Protuberans (DFSP)

Dermatofibrosarcoma protuberans is a rare soft-tissue sarcoma that arises in the dermis and grows slowly but relentlessly into surrounding subcutaneous tissue. It is characterized by a translocation involving the PDGFB gene, leading to overactivation of a growth factor receptor. DFSP rarely metastasizes (fewer than 5% of cases) but has a high local recurrence rate after incomplete surgical excision. It typically presents as a firm, skin-colored or reddish-brown plaque, most commonly on the trunk.

6. Cutaneous T-Cell Lymphoma (CTCL)

Cutaneous T-cell lymphoma is a group of non-Hodgkin lymphomas in which malignant T-lymphocytes primarily involve the skin. The two most common forms are:

• Mycosis fungoides — the most common CTCL, presenting as patches, plaques, and eventually tumors on the skin. It progresses slowly over years to decades.
• Sézary syndrome — an aggressive leukemic variant of CTCL characterized by widespread skin redness (erythroderma), intense itching, and circulating malignant T-cells in the blood.

Other CTCL subtypes include primary cutaneous anaplastic large cell lymphoma, lymphomatoid papulosis, and subcutaneous panniculitis-like T-cell lymphoma.

7. Cutaneous B-Cell Lymphoma

Less common than CTCL, cutaneous B-cell lymphomas arise from malignant B-lymphocytes in the skin. Subtypes include primary cutaneous follicle center lymphoma, primary cutaneous marginal zone lymphoma, and primary cutaneous diffuse large B-cell lymphoma, leg type (the latter being the most aggressive). These present as solitary or grouped nodules, papules, or plaques, typically on the trunk, arms, or legs.

8. Sebaceous Carcinoma

Sebaceous carcinoma is a rare malignant tumor arising from sebaceous (oil) glands in the skin. It most commonly develops on the eyelid (periocular sebaceous carcinoma), where it can mimic benign conditions like chalazion (a blocked gland) or blepharitis (eyelid inflammation), often leading to delayed diagnosis. Extraocular sebaceous carcinoma can arise elsewhere on the head, neck, or trunk. It has significant metastatic potential and can be associated with Muir-Torre syndrome, a hereditary condition involving mismatch repair gene mutations.

9. Eccrine and Apocrine Carcinomas

These are rare malignant tumors arising from sweat glands. Eccrine carcinomas (including microcystic adnexal carcinoma, porocarcinoma, spiradenocarcinoma, and mucinous carcinoma of the skin) tend to grow locally invasively. Apocrine carcinomas most commonly arise in the axilla, groin, or anogenital region. These tumors are often initially mistaken for benign sweat gland tumors and require pathological examination for diagnosis.

10. Trichilemmal and Pilomatrical Carcinoma

These are malignant tumors of the hair follicle. Trichilemmal (tricholemmal) carcinoma arises from the outer root sheath of the hair follicle and most often appears on sun-exposed areas in elderly patients. Pilomatrical carcinoma (malignant pilomatricoma) is an extremely rare tumor arising from hair matrix cells, typically occurring on the head or neck. Both carry risk of local recurrence and, less commonly, metastasis.

11. Angiosarcoma of the Skin

Cutaneous angiosarcoma is a rare and aggressive malignancy arising from the endothelial cells lining blood or lymphatic vessels in the skin. It most commonly affects the scalp and face of elderly individuals, presenting as a bruise-like or violaceous plaque that expands progressively. It can also develop in areas of prior radiation therapy or chronic lymphedema (Stewart-Treves syndrome). Cutaneous angiosarcoma has a poor prognosis due to its tendency for local infiltration and early distant metastasis.

12. Kaposi Sarcoma

Kaposi sarcoma is a vascular tumor caused by human herpesvirus 8 (HHV-8). It presents as red, purple, or brown patches, plaques, or nodules on the skin and mucous membranes. There are four clinical variants: classic (primarily affecting elderly Mediterranean or Eastern European men), endemic (in sub-Saharan Africa), iatrogenic (in immunosuppressed organ transplant recipients), and epidemic (AIDS-related, historically the most common form in the context of HIV infection). Treatment and prognosis depend on the clinical variant and extent of disease.

13. Extramammary Paget’s Disease

Extramammary Paget’s disease (EMPD) is an intraepithelial adenocarcinoma arising in areas rich in apocrine glands, most commonly the vulva, scrotum, perianal region, and axilla. It appears as a red, eczema-like, weeping plaque that does not respond to standard topical treatments. EMPD can be primary (arising in the skin itself) or secondary (a manifestation of an underlying internal adenocarcinoma, such as rectal or bladder carcinoma). It tends to recur locally after excision.

14. Atypical Fibroxanthoma (AFX) and Pleomorphic Dermal Sarcoma (PDS)

Atypical fibroxanthoma is a superficial, low-grade spindle cell tumor that typically arises on sun-damaged skin of the head and neck in elderly patients. It is generally considered a tumor of low malignant potential when completely excised. Pleomorphic dermal sarcoma is considered a deeper, more aggressive counterpart to AFX, with a higher risk of local recurrence and metastasis.

Warning Signs and Symptoms of Skin Cancer

The symptoms of skin cancer vary by type, but several general warning signs apply across most forms.

The ABCDEs of Melanoma

The ABCDE rule is a widely used framework for recognizing potential melanoma:

General Signs of Skin Cancer Across All Types

• A new growth or sore that does not heal. Any open sore, ulcer, or lesion that bleeds easily and fails to heal within a few weeks should be evaluated promptly.
• A pearly or waxy bump. Nodular basal cell carcinoma often presents as a translucent or pearly dome-shaped bump, sometimes with visible blood vessels (telangiectasias) on the surface.
• A flat, flesh-colored or brown scar-like lesion. Morpheaform BCC can resemble scar tissue, making it easy to overlook.
• A red, scaly patch. Squamous cell carcinoma and actinic keratosis (a precancerous lesion) often present as rough, scaly, reddish patches, especially on chronically sun-exposed areas.
• A firm, red nodule. This can be a presentation of SCC or other skin tumors.
• A wart-like growth. SCC can occasionally mimic a wart (verruca), particularly on the hands and around the nails.
• A mole that changes or a new dark spot. Any significant change in an existing mole — or a new, darkly pigmented spot — warrants evaluation for melanoma.
• A nail streak or dark area under the nail. A dark vertical streak in the nail (melanonychia) may be a sign of subungual melanoma, particularly when the stripe is widening or associated with pigment spreading onto the surrounding skin (Hutchinson’s sign).
• A flesh-colored or bluish nodule. Merkel cell carcinoma typically appears as a rapidly growing, firm, painless nodule, often flesh-colored or reddish-blue.
• Persistent itching, tenderness, or pain in a skin lesion, though many skin cancers are asymptomatic in early stages.
• Swollen lymph nodes near a suspicious skin lesion may indicate that cancer has spread to the regional lymphatic system.

Location-Specific Patterns

Skin cancer does not occur only on sun-exposed areas. Melanoma can appear on the scalp, between the toes, on the soles of the feet, beneath the nails, on the genitalia, or inside the mouth or eye. Merkel cell carcinoma predominantly affects the head and neck. Extramammary Paget’s disease appears in the groin or vulvar region. Any new, changing, or unusual growth anywhere on the skin deserves medical attention regardless of sun exposure history.

Where Can Skin Cancer Metastasize?

Metastasis — the spread of cancer beyond its original site — depends heavily on the type and stage of skin cancer. Non-melanoma skin cancers (especially BCC) rarely metastasize, while melanoma, Merkel cell carcinoma, and angiosarcoma can spread aggressively.

Regional Lymph Node Metastasis

The most common initial site of spread for most skin cancers is the regional lymph nodes — the lymph nodes closest to the primary tumor. For example, a melanoma on the arm may first spread to the axillary (underarm) lymph nodes; one on the leg may spread to the inguinal (groin) nodes; and facial melanomas may spread to the cervical (neck) nodes. Detection of lymph node involvement significantly changes staging and treatment planning. The sentinel lymph node biopsy procedure, used primarily in melanoma, involves identifying and sampling the first lymph node(s) draining the tumor site to determine whether microscopic spread has occurred.

Melanoma: Distant Metastasis Sites

Melanoma is notorious for its ability to spread to virtually any organ in the body. Common sites of distant (Stage IV) metastasis include:

• Lungs — one of the most frequently involved organs; pulmonary metastases may cause cough, shortness of breath, or chest pain, but are often detected incidentally on imaging.
• Liver — hepatic metastases may cause abdominal pain, jaundice, or elevated liver enzymes.
• Brain — melanoma has a particularly high propensity for brain metastasis compared to many other cancers; neurological symptoms such as headache, seizures, or cognitive changes may result.
• Bone — bone metastases can cause pain, fractures, or spinal cord compression.
• Gastrointestinal tract — less common but possible, causing bleeding or obstruction.
• Skin and subcutaneous tissue (in-transit metastases) — small satellite tumors may develop in the skin or soft tissue between the primary lesion and the regional lymph nodes, a pattern called in-transit metastasis.
• Adrenal glands and kidneys — less frequently affected.

Squamous Cell Carcinoma: Metastasis Sites

While most SCCs are caught before they spread, high-risk or neglected SCCs can metastasize. Spread follows a similar pattern: first to regional lymph nodes, then potentially to the lungs, liver, brain, bones, and skin. High-risk features that increase metastatic risk include tumors larger than 2 cm, tumors greater than 2 mm thick or invading deeper than the dermis, poorly differentiated histology, perineural invasion (cancer spreading along nerve sheaths), and location on the lip, ear, or in a scar.

Merkel Cell Carcinoma: Metastasis Sites

MCC spreads rapidly and early. It commonly metastasizes to regional lymph nodes at the time of initial diagnosis (approximately 30% of cases). Distant metastases occur most frequently in the liver, lungs, bones, brain, and skin. The 5-year survival rate for distant metastatic MCC is under 20%, underscoring the importance of early detection.

Basal Cell Carcinoma: Rare Metastasis

BCC metastasizes in fewer than 0.1% of cases, typically in the setting of very large, neglected tumors; radiation-associated BCCs; or basosquamous carcinoma (a hybrid subtype). When it does metastasize, it may spread to regional lymph nodes, lungs, bones, and liver.

Other Rare Skin Cancers

Cutaneous angiosarcoma carries an extremely poor prognosis partly because of its tendency to spread to the lungs, liver, and regional lymph nodes. Kaposi sarcoma can spread to the lymph nodes, gastrointestinal tract, and lungs. Sebaceous carcinoma can metastasize to regional lymph nodes and distant organs, particularly in the setting of Muir-Torre syndrome.

Risk Factors for Skin Cancer

Several factors increase the likelihood of developing skin cancer. The most significant include:

• Ultraviolet radiation exposure — from sunlight and tanning beds; cumulative lifetime exposure is a major risk factor for BCC and SCC, while intermittent intense exposure and sunburns are more strongly linked to melanoma.
• Fair skin, light hair, and light eyes — lower melanin content provides less protection against UV radiation.
• History of sunburns — especially blistering sunburns in childhood or adolescence.
• Personal or family history of skin cancer — a prior skin cancer significantly increases the risk of developing another.
• Large number of moles or atypical (dysplastic) moles — individuals with many moles or atypical moles are at increased melanoma risk.
• Immunosuppression — organ transplant recipients and people living with HIV are at substantially elevated risk, particularly for SCC and Merkel cell carcinoma.
• Exposure to carcinogens — arsenic, coal tar, polycyclic aromatic hydrocarbons, and certain pesticides.
• Chronic skin conditions — chronic ulcers, burn scars, or inflammatory skin diseases can predispose to SCC (Marjolin’s ulcer).
• Hereditary syndromes — including xeroderma pigmentosum (impaired DNA repair), Gorlin syndrome (basal cell nevus syndrome, predisposing to multiple BCCs), familial atypical multiple mole melanoma (FAMMM) syndrome, and Li-Fraumeni syndrome.
• Age — most skin cancers occur more frequently with advancing age due to cumulative UV exposure and declining DNA repair capacity, though melanoma is one of the more common cancers in young adults.
• Geographic location and altitude — UV radiation intensity increases closer to the equator and at higher altitudes.

Diagnosis

The diagnostic process for skin cancer typically begins with a thorough visual skin examination by a dermatologist, who may use dermoscopy — a non-invasive technique using a magnifying device with polarized light — to assess the characteristics of a lesion in greater detail. When a suspicious lesion is identified, a skin biopsy is performed. This involves removing all or part of the lesion for histopathological examination under a microscope by a pathologist, who determines the type and, if malignant, the depth of invasion (Breslow thickness for melanoma) and other prognostic features.

For melanoma or other higher-risk cancers, additional staging investigations may include sentinel lymph node biopsy, CT scans, PET scans, MRI, and, for melanoma specifically, molecular testing for mutations such as BRAF V600E, which guides eligibility for targeted therapy.

Treatment Overview

Treatment depends on the type, size, location, depth, and stage of the cancer, as well as the patient’s overall health and preferences.

• Surgical excision — the primary treatment for most skin cancers; involves removing the tumor with a margin of normal surrounding tissue.
• Mohs micrographic surgery — a highly precise surgical technique used for BCCs and SCCs in high-risk locations (face, ears, hands); the surgeon removes tissue layer by layer and examines each layer microscopically until all cancer cells are cleared, maximizing cure rates while minimizing removal of normal tissue.
• Curettage and electrodesiccation (C&E) — used for superficial or small low-risk BCCs and SCCs; the tumor is scraped away and the base is cauterized.
• Radiation therapy — used when surgery is not feasible or as adjuvant treatment after surgery.
• Cryotherapy — liquid nitrogen freezing, commonly used for actinic keratoses and superficial low-risk BCCs.
• Topical treatments — imiquimod and 5-fluorouracil (5-FU) cream are used for superficial BCCs, actinic keratoses, and SCC in situ.
• Targeted therapy — for melanoma, BRAF inhibitors (e.g., vemurafenib, dabrafenib) and MEK inhibitors (e.g., trametinib, cobimetinib) are used in BRAF-mutant metastatic disease; vismodegib and sonidegib target the Hedgehog signaling pathway in advanced BCC; avelumab targets PD-L1 in Merkel cell carcinoma.
• Immunotherapy — checkpoint inhibitors targeting PD-1 (pembrolizumab, nivolumab) and CTLA-4 (ipilimumab) have transformed the treatment of advanced melanoma and are also approved for Merkel cell carcinoma and advanced SCC.
• Systemic chemotherapy — less commonly used for skin cancers but may be employed for advanced or refractory cases.

Prevention

Since UV radiation is the primary risk factor for most skin cancers, sun protection is the cornerstone of prevention:

• Apply broad-spectrum sunscreen (SPF 30 or higher) to all exposed skin, even on cloudy days, and reapply every two hours during prolonged outdoor exposure.
• Wear protective clothing, wide-brimmed hats, and UV-blocking sunglasses.
• Seek shade during peak UV hours (10 a.m. to 4 p.m.).
• Avoid tanning beds and sunlamps entirely.
• Perform regular self-examinations of the skin from head to toe, including between the toes, on the scalp (using a mirror or asking a partner), and under the nails.
• Schedule periodic full-body skin examinations with a dermatologist, especially if you have risk factors such as a large number of moles, a personal or family history of skin cancer, or a history of significant UV exposure.

Frequently Asked Questions

Can skin cancer develop in areas that never see the sun?

Yes. While UV radiation is the predominant cause, skin cancer can develop anywhere on the body, including the palms, soles, beneath the nails, between the toes, the genitalia, and mucous membranes. Acral lentiginous melanoma, for example, specifically arises in non-sun-exposed areas. CTCL, Kaposi sarcoma, and several other skin malignancies are not UV-driven at all.

Is a changing mole always cancer?

Not necessarily. Moles can change for many benign reasons, including hormonal fluctuations, pregnancy, or normal aging. However, any mole that changes in size, shape, or color — or that begins to bleed, itch persistently, or develop new symptoms — should be evaluated by a dermatologist promptly rather than observed at home.

How quickly does skin cancer grow?

Growth rate varies enormously by type. BCC is typically very slow-growing, sometimes taking years to reach a significant size. SCC tends to grow faster than BCC. Nodular melanoma is one of the fastest-growing skin cancers and can develop from a new lesion to a thick, invasive tumor in a matter of weeks to months. Merkel cell carcinoma also grows rapidly. Any rapidly enlarging skin lesion should be seen urgently.

Can skin cancer be cured?

Most skin cancers — particularly BCC and SCC caught at an early stage — have cure rates approaching 95–99% with appropriate treatment. Melanoma has an excellent prognosis when detected at Stage I (localized, thin lesions), with 5-year survival rates above 98%. However, the prognosis for Stage IV (distant metastatic) melanoma, while improved significantly by immunotherapy and targeted therapy in recent years, remains substantially lower. Early detection remains the single most important determinant of outcome for all skin cancers.

Does skin cancer always look like a mole?

No. While melanoma often resembles an abnormal mole, many skin cancers look quite different. BCC may appear as a pearly bump, a flat scar-like lesion, or an open sore. SCC may look like a rough, scaly patch, a firm red bump, or a wart-like growth. Merkel cell carcinoma appears as a flesh-colored or bluish nodule. Any new, unusual, or changing skin growth deserves medical evaluation, regardless of whether it looks like a mole.