Skin Cancer — Basal Cell, Squamous Cell, Melanoma

• UV Radiation and Direct DNA Damage: Ultraviolet B (UVB, 280-320 nm) and ultraviolet A (UVA, 320-400 nm) radiation penetrate the epidermis and dermis, directly damaging cellular DNA through formation of thymine dimers and 6-4 photoproducts. These lesions, if not properly repaired by nucleotide excision repair (NER) mechanisms, result in characteristic mutations including C→T transitions at dipyrimidine sites. Chronic or intermittent sun exposure (particularly sunburning exposure in childhood) overwhelms DNA repair capacity, creating a mutational burden that accumulates over decades. Individuals with defective NER pathways (xeroderma pigmentosum) exhibit >1000-fold increased skin cancer risk, underscoring the critical importance of this repair mechanism.

• p53 Tumor Suppressor Inactivation: TP53 mutations are found in >50% of BCC, >90% of SCC, and >60% of melanomas, representing the most frequent genetic alteration across all skin cancer types. These mutations typically result from UV-induced C→T transitions and ablate p53's ability to induce cell cycle arrest via p21 upregulation or trigger apoptosis through pro-apoptotic gene activation (BAX, PUMA). Loss of p53 function eliminates the critical "guardian of the genome" checkpoint, permitting cells with additional oncogenic mutations to escape senescence and progress toward malignancy. In SCC progression, p53 inactivation often represents a later event facilitating malignant transformation of actinic keratosis (AK) precursors.

• MAPK Pathway Activation and Oncogenic RAS/BRAF Mutations: BCC and melanoma demonstrate distinct patterns of MAPK pathway activation. BCC is characterized by PTCH1 mutations (80-90% of cases) with resultant loss of Hedgehog pathway suppression, leading to constitutive Smoothened (SMO) signaling and aberrant Gliomaassociated oncogene (GLI) transcription factor activation—a pathway distinct from other epithelial malignancies. Melanoma exhibits BRAF V600E mutations in approximately 50% of cases and NRAS mutations in 25-30%, both resulting in constitutive MAPK/ERK pathway activation independent of growth factor signaling. These mutations drive proliferation through sustained phosphorylation of downstream kinases and increased expression of anti-apoptotic proteins (MCL1, BCL-XL).

• Immunosuppression and Immune Evasion: Chronic UV exposure induces immunosuppression through multiple mechanisms including: (1) langerhans cell depletion in the epidermis with altered antigen presentation, (2) IL-10 production by UV-damaged keratinocytes and tumor-infiltrating lymphocytes that suppresses Th1 responses, (3) PD-L1 expression on tumor cells and tumor-associated immune cells that facilitates immune escape, and (4) regulatory T cell (Treg) expansion within the tumor microenvironment. This immunologically permissive microenvironment is particularly critical for melanoma progression, as these tumors arise from melanocytes that are inherently immunogenic due to shared tumor-associated antigens (MART-1, gp100, tyrosinase). Cutaneous T-cell lymphomas (mycosis fungoides) can develop as a consequence of chronic UV-induced immunosuppression, though this represents a distinct neoplastic process.

• Additional Molecular Events in Progression: BCC rarely metastasizes due to early clinical detection of locally advanced disease; however, metastatic potential increases with loss of CDH1 (E-cadherin) and increased expression of epithelial-to-mesenchymal transition (EMT) markers. SCC progression from AK involves sequential p53 mutations followed by loss of p16/RB function (via HPV infection in some cases) and activation of PI3K/AKT pathway. Melanoma progression follows the "Clark model" with evolution from benign nevi (BRAF or NRAS mutation alone insufficient for malignancy due to CDKN2A/p16 checkpoint) to dysplastic nevi to radial growth phase (RGP) melanoma (low metastatic potential) to vertical growth phase (VGP) melanoma (high metastatic potential), with each transition acquiring additional oncogenic lesions including loss of PTEN, TP53, or CDKN2A.

• Ultraviolet Radiation Exposure (Primary Cause): Cumulative and intermittent sun exposure represents the dominant etiologic factor for BCC and SCC, with quantifiable dose-response relationships. Intermittent, intense sun exposure with history of sunburns—particularly during childhood and young adulthood—confers higher melanoma risk than equivalent cumulative exposure, suggesting a developmental window of heightened susceptibility. Solar elastosis (solar degeneration) seen histopathologically in sun-exposed skin serves as a marker of chronically photodamaged tissue at elevated risk for malignant transformation. Artificial UV exposure (tanning beds, PUVA therapy) similarly increases risk, with the International Agency for Research on Cancer (IARC) classifying tanning bed use as a Group 1 carcinogen.

• Genetic Predisposition and Familial Syndromes: Familial atypical multiple mole melanoma (FAMMM) syndrome (also called dysplastic nevus syndrome or familial melanoma) accounts for 5-10% of melanomas and is characterized by germline CDKN2A mutations (p16 mutations in >80% of hereditary melanoma families), conferring 40-70% lifetime melanoma risk. Xeroderma pigmentosum (XP), a rare autosomal recessive condition affecting nucleotide excision repair, results in >1000-fold increased risk of skin cancer with median age of onset in childhood. Gorlin syndrome (nevoid basal cell carcinoma syndrome), caused by PTCH1 germline mutations, predisposes to multiple BCCs (often >100 in a lifetime) and other malignancies (medulloblastoma, ovarian cancer). Individuals with Li-Fraumeni syndrome (TP53 mutations) demonstrate increased melanoma and SCC risk alongside multiple other cancers. Fair skin phenotype (Fitzpatrick types I-II), light hair color, and inability to tan efficiently increase NMSC risk 10-100 fold compared to dark-skinned populations, though melanoma can occur at higher mortality rates in darker-skinned individuals due to delayed diagnosis.

• Immunosuppression: Solid organ transplant recipients on chronic immunosuppressive therapy demonstrate 65-250 fold increased SCC risk and 10-25 fold increased BCC risk, with cutaneous SCC being the most common malignancy post-transplant. HIV/AIDS patients with CD4+ counts <50 cells/μL show markedly elevated risk of Kaposi sarcoma (HHV8-driven), but modest increases in conventional skin cancers. Chronic lymphocytic leukemia (CLL) and lymphoma patients are at increased risk for secondary skin malignancies, particularly SCC.

• Environmental and Occupational Exposures: Arsenic exposure (from pesticides, contaminated groundwater, or occupational sources) increases risk of both BCC and SCC with a threshold-dependent relationship. Exposure to polycyclic aromatic hydrocarbons (PAHs), coal tar, and pitch historically increased SCC risk among workers. Ionizing radiation exposure (from prior radiotherapy, nuclear accidents, or occupational exposure) increases risk of all skin cancer types with a latency period of 5-40 years.

• Chronic Inflammatory Conditions and Ulcers: Chronic wounds, ulcers, and burn scars (Marjolin ulcer) predispose to SCC through sustained inflammation, tissue remodeling, and loss of growth control mechanisms. Chronic inflammatory dermatoses including psoriasis, lichen planus, and lichen sclerosus may be associated with increased SCC risk, though causality remains debated. Severe actinic keratosis represents an intermediate lesion in the progression to SCC, with approximately 2.6% of AK lesions progressing to invasive SCC within 1 year in some populations, though the "field cancerization" phenomenon suggests multiple lesions may be present simultaneously at varying stages of malignant transformation.

• Human Papillomavirus (HPV) Infection: HPV (particularly types 16 and 18) increases SCC risk, particularly in immunocompromised patients and genital SCC. The association is stronger for genital and perianal SCC than for cutaneous SCC in sun-exposed sites.

• Basal Cell Carcinoma—Cardinal Presentation: The classic presentation is a slowly growing, pearly, flesh-colored or translucent papule or nodule with a rolled border, central ulceration (rodent ulcer), and visible telangiectasia (dilated superficial blood vessels). The characteristic "waxy" appearance reflects the basophilic (blue-staining) nature of the neoplastic cells comprising >95% of the lesion. Bleeding or oozing with minor trauma is common and may prompt patient concern. Most lesions remain asymptomatic unless secondarily infected; pain is not a typical feature. BCC typically appears on sun-exposed skin, with the face (especially around eyes and nose) being the most common site, followed by ears, neck, and trunk. The lesion may remain unchanged for years or slowly enlarge at centimeters per year without treatment.

• Squamous Cell Carcinoma—Clinical Variants: SCC presents as a firm, red to flesh-colored nodule, plaque, or ulcer with an indurated border, often with scale, keratin, or crusting on the surface. The lesion may be painful or tender, particularly if ulcerated. Rapid growth over weeks to months is more common with SCC than BCC and may indicate higher-risk disease. SCC frequently arises from a visibly hyperkeratotic, scaly patch (actinic keratosis) in sun-exposed areas. Important clinical subtypes include: (1) well-differentiated SCC with abundant keratinization and slower growth, (2) poorly differentiated SCC with rapid growth and higher metastatic potential, (3) spindle cell SCC appearing as a solitary nodule with aggressive behavior, (4) adenosquamous SCC with glandular differentiation and poor prognosis, and (5) acantholytic SCC with loss of intercellular cohesion and histopathologic mimicry of Paget disease. Lesions typically develop on sun-exposed sites (face, ears, forearms, dorsal hands) but can arise on non-sun-exposed skin in immunocompromised patients. Lip vermillion SCC (lower lip more common than upper) carries higher metastatic potential than cutaneous SCC.

• Melanoma—Presentation and ABCDE Criteria: Melanoma classically presents as an asymmetric, irregularly bordered lesion with variegated color (brown, black, red, white) on the skin of middle-aged to older adults, though it can occur at any age. The ABCDE mnemonic captures warning features: (A)symmetry (one half does not match the other), (B)order irregularity (irregular, notched, or scalloped margins), (C)olor variegation (multiple colors or uneven pigmentation), (D)iameter >6 mm (though some melanomas are smaller), and (E)volution (recent change in size, shape, or color). The lesion typically exhibits rapid growth over weeks to months, which distinguishes it from benign nevi. Melanomas can develop de novo from normal-appearing skin or arise from a pre-existing nevus. Clinical variants include:
• Superficial spreading melanoma (SSM) (~70% of melanomas): presents as an irregularly bordered, variegated macule or patch with radial growth phase
• Nodular melanoma (~15%): a rapidly enlarging, darkly pigmented papule or nodule with rapid vertical growth and worse prognosis than SSM
• Lentigo maligna melanoma (~10%): arises from lentigo maligna (in-situ melanoma) on the face of elderly patients; initially flat but may develop a dark nodule indicating invasive transformation
• Acral lentiginous melanoma (~5% in Caucasians, >50% in darker-skinned populations): occurs on palms, soles, or nail beds; often presents late due to anatomic location and lower clinical suspicion in non-Caucasian patients
• Desmoplastic melanoma: rare variant with spindle cells and fibroblastic stromal response; may lack pigmentation (amelanotic) and present as a flesh-colored or gray nodule; high local recurrence rate
• Amelanotic melanoma: lacks melanin pigment, appearing pink or flesh-colored; often diagnosed at advanced stages due to lack of pigmentation raising clinical suspicion

• Physical Examination Findings: BCC and SCC typically present as solitary lesions, though patients with significant sun exposure may have multiple lesions. A careful full-body skin examination should assess for additional suspicious lesions, particularly in elderly patients with significant sun exposure history. The surrounding skin should be assessed for actinic keratosis, solar lentigines, and solar elastosis indicating field cancerization. Palpation assesses for induration, fixation to underlying structures (suggesting more advanced disease), and regional lymphadenopathy (indicating lymph node involvement). Melanoma examination should document the lesion characteristics (size, color, borders, elevation), presence of satellite lesions (separate tumors within 2 cm), and assessment of regional lymph nodes. Dermoscopy (examination with 10x magnification ± oil immersion) reveals characteristic patterns: BCC shows arborizing vessels and leaf-like areas; SCC shows keratin, ulceration, and polymorphous vessels; melanoma shows asymmetric pigment distribution, irregular dots/globules, streaks, and atypical branching patterns.

• Systemic Symptoms Indicating Advanced Disease: Most patients with early skin cancer are asymptomatic. Pain, ulceration with discharge or bleeding, lymphadenopathy, constitutional symptoms (weight loss, fatigue), or palpable skin nodules in distant sites suggest locally advanced or metastatic disease requiring urgent staging and intervention. Pruritus within or around a lesion may indicate progression or malignant transformation of a pre-existing lesion.

• Clinical Diagnosis and Dermoscopy: The clinical diagnosis of skin cancer relies on recognition of characteristic morphologic features supplemented by dermoscopy. Dermoscopy is a non-invasive technique using 10x magnification (with or without oil immersion or polarizing light) that enhances visualization of structural and vascular patterns. For BCC, dermoscopy demonstrates: arborizing (tree-like branching) telangiectatic vessels, blue-gray ovoid nests, leaf-like areas of hyperpigmentation, and shiny white streaks (fibrosis). For SCC, dermoscopy shows: keratin, disruption of normal skin markings, polymorphous vessels (dotted, linear, branching), and ulceration. For melanoma, dermoscopy employs the ABCDE and 7-point criteria: asymmetry, irregular borders, color variegation, diameter, evolution (ABCDEs), plus atypical network, blue-white veil, and irregular dots/globules (7-point criteria). Dermoscopy has 87-97% sensitivity and 79-98% specificity for melanoma detection when used by trained examiners, improving diagnostic accuracy compared to clinical examination alone (70-90% sensitivity

Basal and squamous cell carcinoma (NCCN Guidelines for Basal Cell/Squamous Cell Skin Cancer; AAD guidelines of care)

• Low-risk, non-facial lesions: standard excision with narrow clinical margins, or electrodesiccation and curettage — avoided in hair-bearing sites, where follicular extension escapes the curette.
• Superficial BCC and actinic keratoses: topical antineoplastics/immune modulators — 5-fluorouracil or imiquimod; cryotherapy or photodynamic therapy for field cancerization.
• High-risk lesions (face, "H-zone," recurrent, morpheaform/infiltrative, perineural invasion, poorly differentiated, immunosuppressed host): Mohs micrographic surgery, which gives complete circumferential peripheral and deep margin assessment with maximal tissue conservation, per the AAD/ACMS/ASDSA/ASMS appropriate use criteria.
• Non-surgical candidates: definitive radiotherapy.
• Escalation: hedgehog pathway inhibitors (vismodegib) for locally advanced/metastatic BCC — mechanistically targeting SMO downstream of PTCH1 loss; anti-PD-1 therapy (cemiplimab) for advanced cutaneous SCC and for BCC after hedgehog inhibitor failure.

Melanoma (NCCN; ASCO–SSO sentinel lymph node guideline)

• Wide local excision with margins set by Breslow depth: about 0.5–1 cm for in situ, 1 cm for ≤1 mm, 1–2 cm for >1–2 mm, and 2 cm for >2 mm.
• Sentinel lymph node biopsy: discussed for thin lesions with adverse features (≈0.8–1.0 mm or ulcerated) and recommended for intermediate-thickness melanoma; it is staging, not therapeutic.
• Advanced/adjuvant: checkpoint inhibitors (pembrolizumab, nivolumab ± ipilimumab) and, for BRAF V600-mutant tumors, BRAF + MEK inhibitors (dabrafenib/trametinib).

Disease-related

• Local destructive invasion (BCC): indolent but relentless growth erodes cartilage, bone, and orbit — the rodent ulcer. Morpheaform/infiltrative subtypes have subclinical extension far beyond the visible edge; the signal is a scar-like plaque with ill-defined borders or repeated recurrence after excision.
• Perineural invasion (SCC > BCC): tumor tracks along nerve sheaths, classically the trigeminal and facial nerves on the mid-face. The finding that betrays it is facial numbness, paresthesia, formication, or new facial weakness in a patient with a periorbital or perinasal lesion — obtain MRI; this is high-risk disease requiring Mohs and consideration of adjuvant radiation.
• Regional nodal and distant metastasis (SCC, melanoma): cSCC metastasizes preferentially from lip, ear, Marjolin ulcer, and immunosuppressed hosts; new firm, fixed regional lymphadenopathy is the sentinel finding. Melanoma disseminates hematogenously to lung, liver, bone, and brain.
• Brain metastasis — emergency: melanoma metastases are characteristically hemorrhagic; headache, seizure, or focal deficit warrants emergent neuroimaging and steroids for vasogenic edema.

Treatment-related

• Surgical: scar contracture, ectropion after periorbital excision, motor nerve injury; lymphedema and wound infection after complete lymph node dissection.
• Radiotherapy: chronic radiodermatitis, ulceration, and second cutaneous malignancies within the field after long latency.
• Topical 5-FU/imiquimod: brisk erosive inflammatory reaction — expected, not failure; distinguish from secondary bacterial infection.
• Hedgehog inhibitors: muscle spasms, dysgeusia, alopecia, weight loss, and embryo-fetal toxicity.
• Checkpoint inhibitors: immune-related adverse events from loss of peripheral tolerance — colitis (bloody diarrhea; perforation is an emergency), hepatitis, pneumonitis, thyroiditis, hypophysitis, and fulminant myocarditis, an emergency presenting with new arrhythmia, heart failure, and elevated troponin. Management is drug hold plus high-dose systemic corticosteroids. Vitiligo-like depigmentation is a favorable prognostic sign.
• BRAF inhibitor monotherapy: paradoxical MAPK activation in RAS-mutant keratinocytes produces new keratoacanthomas and SCCs — the reason a MEK inhibitor is added.

• Breslow depth is the single most important prognostic factor in localized melanoma, and it drives excision margins and the decision to offer sentinel node biopsy. Clark level is the classic distractor — it was dropped from AJCC staging.
• Best next step for a pigmented lesion suspicious for melanoma: narrow-margin excisional biopsy through the subcutaneous fat. Choosing a superficial shave, a partial punch of a large lesion, cryotherapy, or "observe and repeat in 6 months" is wrong because depth cannot be assessed.
• Histology buzzwords: BCC shows peripheral palisading of nuclei with retraction (clefting) artifact; well-differentiated SCC shows keratin pearls and intercellular bridges; melanoma stains for S-100 (most sensitive), SOX10, HMB-45, and Melan-A/MART-1.
• Nail and acral disease: a longitudinal pigmented band with pigment spilling onto the proximal nail fold (Hutchinson sign) is subungual melanoma until proven otherwise. Acral lentiginous melanoma is not more common in darkly pigmented patients in absolute terms — it is the most common subtype in that group, and outcomes are worse because of delayed diagnosis.
• The association examiners love: solid organ transplant recipients on chronic immunosuppression develop SCC more often than BCC — a reversal of the normal ratio — and these SCCs behave aggressively.
• Marjolin ulcer: SCC arising in a chronic burn scar, sinus tract, or non-healing wound; a wound that suddenly heaps up, everts, or bleeds after years of stability should be biopsied.
• **The *ugly duckling sign*** — a nevus that looks unlike the patient's other nevi — outperforms rigid ABCDE application, particularly for nodular and amelanotic melanoma, which may be symmetric, uniformly pink, and <6 mm.
• Screening: the USPSTF concludes that current evidence is insufficient to assess the balance of benefits and harms of routine whole-body skin cancer screening in asymptomatic adults (I statement) — do not select "annual total-body screening for everyone" as the guideline-supported answer.