Skin Cancer: Melanoma, Basal Cell, and Squamous Cell

Skin cancer is the most commonly diagnosed cancer in the United States, with the American Cancer Society estimating more than 5 million cases treated annually across all types combined. This page covers the three principal forms — melanoma, basal cell carcinoma, and squamous cell carcinoma — their mechanisms, distinguishing characteristics, and the clinical and regulatory frameworks that govern their detection and management. Understanding how these cancers differ from one another is essential for interpreting screening recommendations, pathology reports, and treatment pathways within oncology practice.

Definition and scope

The National Cancer Institute (NCI) classifies skin cancer by the cell type of origin within the epidermis. Three diagnoses account for the overwhelming majority of cases:

Basal cell carcinoma (BCC): Arises from basal cells in the deepest layer of the epidermis. The American Cancer Society reports BCC constitutes roughly 80 percent of all skin cancers diagnosed in the United States (American Cancer Society, Skin Cancer Facts).
Squamous cell carcinoma (SCC): Originates in squamous cells in the outer layers of the skin. SCC accounts for approximately 20 percent of skin cancer diagnoses and carries a meaningfully higher metastatic risk than BCC.
Melanoma: Arises from melanocytes, the pigment-producing cells. Although melanoma represents a smaller fraction of total skin cancer diagnoses — roughly 1 percent by case count — it is responsible for approximately 80 percent of skin cancer deaths (National Cancer Institute SEER Program).

Skin cancer falls under the regulatory and clinical oversight frameworks described in regulatory context for oncology, including FDA-cleared diagnostic tools, CLIA-regulated pathology laboratories, and CMS reimbursement codes for dermatoscopy and Mohs surgery. The U.S. Preventive Services Task Force (USPSTF) addresses behavioral counseling for skin cancer prevention in its published evidence reviews.

How it works

Each of the three major skin cancers follows a distinct pathogenic pathway tied to its cell of origin.

Basal cell carcinoma develops when cumulative ultraviolet (UV) radiation — primarily UVB, wavelength 280–315 nanometers — damages the PTCH1 tumor suppressor gene on chromosome 9q22, disabling the Hedgehog signaling pathway. Without intact suppression, basal cells proliferate abnormally. BCC rarely metastasizes (fewer than 0.1 percent of cases) but can invade surrounding tissue aggressively if untreated over time.

Squamous cell carcinoma follows UV-induced mutation of the TP53 tumor suppressor gene. Chronic exposure progressively accumulates TP53 mutations in keratinocytes, leading to dysplasia and eventual invasive carcinoma. Actinic keratoses — rough, scaly precancerous patches — are recognized precursor lesions. SCC has a metastatic rate of approximately 2–5 percent in cutaneous cases, rising substantially in immunocompromised patients or tumors arising on the lip or ear.

Melanoma is driven by mutations in oncogenes including BRAF (mutated in approximately 50 percent of cutaneous melanomas), NRAS, and CDKN2A. UV radiation initiates and promotes these mutations in melanocytes. Melanoma's capacity for early hematogenous and lymphatic spread makes tumor thickness at diagnosis — measured using the Breslow depth in millimeters — the single strongest predictor of prognosis. The American Joint Committee on Cancer (AJCC) 8th Edition staging system formalizes Breslow depth thresholds at 1.0 mm and 2.0 mm as key boundaries within the T-category classification.

Common scenarios

Skin cancer presents across a range of clinical and demographic contexts. The following represent the most frequently encountered diagnostic scenarios:

Chronic sun-exposed skin in older adults: BCC and SCC appear most commonly on the face, ears, neck, and dorsal hands of individuals with decades of cumulative UV exposure. Lesions often present as slow-growing, pearly, or scaly patches.
Younger adults with intermittent intense exposure: Melanoma incidence is disproportionately higher in individuals who experienced blistering sunburns in childhood or adolescence, even without chronic daily exposure.
Immunosuppressed patients: Organ transplant recipients on calcineurin inhibitors face a 65–250 times higher risk of developing SCC compared with the general population, a figure documented in transplant dermatology literature reviewed by the International Transplant Skin Cancer Collaborative (ITSCC).
Genetic predisposition: Individuals with basal cell nevus syndrome (Gorlin syndrome) carry germline PTCH1 mutations and may develop hundreds of BCCs before age 20. Familial atypical multiple mole melanoma (FAMMM) syndrome elevates lifetime melanoma risk substantially and warrants genetic testing and counseling.
Changing or atypical pigmented lesions: Melanoma is evaluated using the ABCDE criteria (Asymmetry, Border irregularity, Color variation, Diameter greater than 6 mm, Evolution) established in clinical dermatology and adopted in NCI patient education materials.

Decision boundaries

Distinguishing among the three skin cancer types — and from benign lesions — requires structured clinical and pathological criteria. The following contrasts define the primary decision boundaries in practice:

BCC vs. SCC: Both are non-melanoma skin cancers (NMSCs), but their risk profiles diverge. BCC rarely spreads beyond local tissue. SCC can metastasize, particularly lesions exceeding 2 cm in diameter, those with perineural invasion on pathology, and those located on the lip or external ear. SCC also warrants sentinel lymph node evaluation in high-risk cases under NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines, Squamous Cell Skin Cancer, available at nccn.org).

Melanoma in situ vs. invasive melanoma: The presence or absence of dermal invasion on biopsy determines staging. Melanoma confined to the epidermis (in situ) carries a near-100 percent 5-year survival rate. Breslow depth greater than 4.0 mm places the tumor in T4 classification under AJCC 8th Edition staging, correlating with significantly reduced survival probability.

Actinic keratosis vs. SCC: Actinic keratoses are not carcinomas but represent a spectrum that progresses to invasive SCC in an estimated 0.025–16 percent of untreated lesions annually, depending on the study population and follow-up methodology (data summarized by the NCI).

Nodular melanoma vs. superficial spreading melanoma: Superficial spreading melanoma — the most common subtype, comprising approximately 70 percent of melanomas — grows radially before vertical invasion. Nodular melanoma, roughly 15–20 percent of cases, lacks a prolonged radial growth phase and reaches invasive depth more rapidly, creating a compressed diagnostic window.

Treatment pathway selection — whether surgical excision, Mohs micrographic surgery, radiation, immunotherapy with checkpoint inhibitors such as pembrolizumab, or BRAF-targeted therapy — follows staging and histologic subtype, as addressed in detail across chemotherapy, immunotherapy, and targeted therapy reference pages.

References

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