Non-Melanoma Skin Cancer Pathology — BCC and SCC

Solar UV Radiation and DNA Damage

• UVB radiation (280-320 nm) penetrates the epidermis and causes direct DNA damage through thymine dimer formation, primarily affecting TP53 tumor suppressor gene, occurring in up to 90% of BCC and 50-70% of SCC
• UVA radiation (320-400 nm) generates reactive oxygen species (ROS) causing oxidative DNA damage and indirect mutagenesis
• Cumulative UV exposure rather than intense intermittent exposure correlates more strongly with BCC development, whereas SCC risk increases with both cumulative dose and history of sunburns
• Impaired nucleotide excision repair (NER) mechanisms (as seen in xeroderma pigmentosum) dramatically increase NMSC susceptibility

Molecular Oncogenesis Pathways

• BCC pathogenesis: Loss of PTCH1 (patched homolog 1) tumor suppressor on chromosome 9q22 leads to constitutive hedgehog (Hh) signaling activation, resulting in aberrant GLI1/GLI2 transcription factor activity; TP53 mutations occur secondarily; PTCH1 mutations occur in ~85% of sporadic BCC and in hereditary syndromes (Nevoid BCC syndrome/Gorlin syndrome with autosomal dominant inheritance)
• SCC pathogenesis: Sequential accumulation of mutations in TP53 (early/initiating), CDKN2A/p16 (cell cycle checkpoint loss), NOTCH1 (differentiation pathway disruption), and HRAS oncogene activation; inflammation and immunosuppression create permissive microenvironment
• PIK3CA mutations enhance growth signaling in both BCC and SCC
• Loss of p53 function abolishes apoptosis in response to UV damage, permitting clonal expansion of damaged keratinocytes

Cellular and Tissue-Level Changes

• Hedgehog signaling hyperactivation in BCC drives proliferation of primitive keratinocytes with mesenchymal-like features and suppresses differentiation
• Desmoplastic stromal response characterized by fibroblast proliferation and increased collagen deposition, particularly prominent in infiltrative BCC variants
• Altered keratinocyte differentiation in SCC with loss of normal terminal differentiation markers and increased expression of proliferation-associated antigens
• Immunosuppression via UV-induced Langerhans cell dysfunction and induction of regulatory T cells (Tregs), allowing escape from immune surveillance

Major Environmental Risk Factors

• Chronic UV exposure (cumulative for BCC; intermittent/acute for SCC): The most significant modifiable risk factor; 80-90% of NMSCs attributable to solar radiation
• Ionizing radiation exposure: Associated with significantly increased NMSC risk; latency period typically 15-40 years; includes therapeutic radiation (e.g., for prior cutaneous lymphoma or breast cancer), occupational exposure, or atomic bomb radiation
• Chemical carcinogens: Arsenic exposure (occupational or medicinal), polycyclic aromatic hydrocarbons (PAHs), and tar exposure increase SCC risk; arsenic demonstrates dose-response relationship and can cause multiple NMSCs decades after exposure

Host and Genetic Risk Factors

• Skin phototype: Fair skin (Fitzpatrick types I-II), red hair, and inability to tan (due to reduced melanin photoprotection) carry 15-50× higher risk than darker skin types
• Age: Peak incidence 60-80 years; cumulative lifetime UV exposure is primary driver
• Immunosuppression: Solid organ transplant recipients demonstrate 40-250× increased NMSC risk (with SCC predominating); HIV/AIDS patients show elevated incidence; chronic corticosteroid or other immunosuppressive therapy increases risk
• Hereditary syndromes:
• Nevoid BCC syndrome (Gorlin syndrome): Autosomal dominant PTCH1 mutations; develops hundreds of BCCs by adulthood
• Xeroderma pigmentosum: Autosomal recessive NER defects; extreme photosensitivity and >1000× NMSC risk
• Oculocutaneous albinism: Reduced melanin protection increases SCC risk
• CDKN2A mutations (melanoma-prone families): Increased both melanoma and NMSC risk
• Prior NMSC: Approximately 40% of patients with one NMSC develop another within 5 years (field cancerization)
• Chronic inflammatory or scarring conditions: Chronic wounds, burn scars, radiation dermatitis, lichen sclerosus, and lupus vulgaris (tuberculosis of skin) significantly increase SCC risk through chronic irritation and impaired healing

Occupational and Behavioral Exposures

• Sunbed/tanning bed use: Associated with 15% increased NMSC risk, particularly SCC
• Smoking: Modest increase in both BCC and SCC risk
• Outdoor occupations: Construction, farming, landscaping workers demonstrate 3-5× elevated incidence

Basal Cell Carcinoma Presentation

• Painless, slow-growing nodule over months to years; most patients initially perceive as cosmetic concern rather than dangerous lesion
• Pearly, translucent appearance with telangiectatic vessels visible (tortuous dilated capillaries)—classic "rodent ulcer" descriptor reflects both pearly center and ulcerated border
• Central ulceration with rolled, waxy borders (typical presentation); surrounding erythema or pigmentation variable
• Bleeding or crusting following minor trauma or picking, followed by recurrence of lesion (does not heal completely)
• Location: 80% on head and neck (especially periocular, perinasal areas); higher risk lesions occur on high-risk facial zones (central face, ears, lips)

• Nodular BCC (60% of cases): Classic pearly nodule with central ulceration and rolled borders; most common presentation
• Superficial BCC (15-35%): Ill-defined erythematous patch or thin plaque, often mistaken for eczema or psoriasis; occurs on trunk; slow growth over years
• Infiltrative/morpheaform BCC (5-10%): Scar-like plaque with indistinct borders, often pale or flesh-colored; highest recurrence rate (20-40%) due to subclinical extension
• Pigmented BCC (10-20%): Dark brown to black coloration (from melanin, not melanoma) mimicking melanoma; common in darker-skinned individuals and Mediterranean populations
• Basosquamous BCC (<5%): Aggressive variant with squamous differentiation; higher recurrence and metastatic potential

Squamous Cell Carcinoma Presentation

• Firm, hyperkeratotic nodule or plaque with crusted or scaly surface; often tender or symptomatic (unlike BCC)
• Rapid growth over weeks to months (faster than BCC); bleeding, ulceration, and pain suggest more aggressive behavior
• Indurated, irregular borders with surrounding erythema; may have granular base
• Origin from actinic keratosis (AK): 60% of SCC arise from pre-existing AK (premalignant dysplasia); differentiation from AK may be histological (depth of invasion) rather than clinical
• Location: Sun-exposed areas, particularly ears, lip vermillion (higher-risk area), scalp, dorsal hands; increased risk on lower lip and anterior ear

• Well-differentiated SCC (low-grade, <2 mm depth): Slow growth, <2% metastatic risk; keratin horn may be prominent clinically
• Moderately differentiated SCC (2-4 mm depth): Intermediate behavior; 5-10% metastatic risk
• Poorly differentiated SCC (>4 mm depth, high-grade): Rapid growth, often ulcerated and bleeding; up to 45% regional lymph node involvement; significantly higher metastatic potential
• Verrucous carcinoma variant (SCC subtype): Locally aggressive with minimal metastatic potential; massive hyperkeratotic lesion mimicking wart; occurs intraorally or anogenitally
• Spindle cell carcinoma variant: Aggressive spindle cell morphology; high recurrence and metastatic rates
• Adenosquamous carcinoma: Rare, aggressive variant with glandular differentiation; associated with poor prognosis

Overlapping Clinical Features

• Field cancerization: Multiple NMSCs and actinic keratoses in same anatomic area reflecting widespread cumulative UV damage; typical patient presents with 3-6 visible lesions simultaneously
• Lack of constitutional symptoms: NMSCs rarely present with systemic manifestations unless advanced/metastatic
• Sensory symptoms: SCC more likely to be symptomatic (pain, tenderness, bleeding) compared to asymptomatic BCC
• No pathognomonic laboratory abnormalities: Diagnosis requires tissue confirmation; no tumor markers with clinical utility

Clinical Examination and Risk Stratification

• Careful inspection of entire skin surface for multiple lesions and field cancerization; higher-risk anatomic sites (ears, lip, temporal scalp) warrant heightened clinical suspicion and lower threshold for biopsy
• Palpation to assess induration, depth, and fixation to underlying structures; infiltrative lesions may feel firmer/scarred
• Examination of regional lymph nodes to assess for metastatic involvement; SCC >4 mm, poorly differentiated, or >20 mm diameter warrant nodal staging
• Dermoscopy: BCC typically shows arborizing vessels (branching tree-like pattern), blue-gray ovoid nests, and retraction at periphery; SCC shows glomerular vessels, keratin masses, and infiltrative borders

Histopathological Diagnosis (Gold Standard)

• Nests and strands of basaloid cells with darkly staining nuclei (high nuclear-to-cytoplasmic ratio) arranged in orderly islands surrounded by myxoid stroma
• Peripheral palisading: Characteristic outer layer of nuclei oriented perpendicular to basement membrane (columnar arrangement at periphery)
• Retraction artifact: Clear space between tumor nests and surrounding stroma (helpful distinguishing feature; occurs because basaloid nests lack true intercellular bridges unlike normal epidermis)
• Loss of peripheral basement membrane demarcating tumor from dermis in infiltrative variants
• Intact overlying epidermis in non-ulcerated lesions; ulceration visible as loss of epidermis with granulation tissue and inflammation in base
• Tumor-associated stroma: Fibroblasts, myxoid material, and variable inflammation; desmoplastic (fibrotic) response more prominent in infiltrative variants
• Mucin/hyaluronic acid: Often visible in stroma as light blue material with special stains
• No keratinization or horn pearls (distinguishing BCC from well-differentiated SCC)
• Immunohistochemistry:
• Positive: BerEP4, EMA, SOX9 (helpful in superficial lesions)
• CD34+ fibroblasts surrounding nests (characteristic peritumoral fibroblast phenotype)
• Negative: P63 expression less organized than in SCC

• Nests and infiltrative strands of atypical squamous cells with prominent keratin pearl formation (concentric whorls of keratin in center of nests—pathognomonic for SCC)
• High nuclear-to-cytoplasmic ratio with hyperchromatic nuclei, coarse chromatin, and abnormal mitotic figures (including atypical forms)
• Desmoplastic stromal response with dense fibroblast infiltration and collagen deposition; variable lymphocytic inflammation
• Depth of invasion: Critical prognostic factor; Clark level and Breslow depth (measured from granular layer to deepest invasion) guide staging and management
• Level I (in situ): Confined to epidermis (carcinoma in situ/Bowen disease); essentially zero metastatic risk
• Level II: Invasion into papillary dermis; minimal metastatic risk (<5%)
• Level III: Invasion to mid-dermis; increased metastatic risk (5-15%)
• Level IV: Invasion to reticular dermis; significantly increased risk (15-30%)
• Level V: Invasion into subcutaneous fat/muscle; very high metastatic risk (>30%)
• Degree of differentiation (Broders' grade):
• Grade I (Well-differentiated): Abundant keratinization, intact tumor architecture, minimal nuclear pleomorphism; most keratin, least atypical nuclei
• Grade II (Moderately differentiated): Moderate keratinization, mixed architecture, moderate atypia
• Grade III (Poorly differentiated): Minimal keratinization, disorganized nests, marked atypia, high mitotic rate; minimal keratin, maximal atypia
• Grade IV (Undifferentiated): No evidence of squamous differentiation; spindle or small round cell morphology; worst prognosis
• Transition from actinic keratosis: Upper lesions often show graded dysplasia (actinic keratosis-like changes) with invasive component below
• Immunohistochemistry:
• P63+ (nuclear staining; marks basal/parabasal cells; helps confirm squamous origin in poorly differentiated lesions)
• Cytokeratins (AE1/AE3)+ in squamous nests
• CD34+ fibroblasts less organized peritumoral pattern than BCC
• Negative: BerEP4

Gross Pathology

• BCC specimen: Small nodule with pearly, translucent surface; centrally ulcerated lesions show crater-like defect with waxy rolled borders; cut surface shows gray-white to tan tumor tissue; nodular variants appear as circumscribed mass, infiltrative variants show poorly demarcated, scar-like tissue
• SCC specimen: Nodular or plaque-like lesion with keratotic/scaly surface; ill-defined infiltrative margins; cut surface tan-white with variable amounts of keratin visible as gritty material; ulcerated lesions show granulation tissue and necrotic debris

Diagnostic Imaging and Staging

• Dermoscopy: Helpful adjunct for clinical diagnosis but cannot replace histology
• High-frequency ultrasound: Emerging modality for assessing invasion depth and margins; useful in large lesions to measure thickness
• MRI/CT: Reserved for advanced/metastatic disease or lesions with suspicion of deep invasion into bone or nerve
• Sentinel lymph node biopsy (SLNB): Considered for SCC with high-risk features:
• Grade III

Before treating — establish tissue diagnosis

• Biopsy first: a shave (BCC, superficial SCC) or punch/incisional biopsy (suspected deep or infiltrative lesion) precedes any destructive therapy, because margin planning depends on subtype and depth. NCCN Guidelines for Basal Cell and Squamous Cell Skin Cancer stratify every lesion as low-risk or high-risk before choosing a modality.
• Urgent situations are rare but include brisk hemorrhage from a tumor eroding a superficial vessel, and orbital or intracranial extension — these need immediate hemostasis and surgical/oncologic referral rather than office destruction.

Low-risk lesions (trunk/extremities, small, well-defined, primary)

• Standard surgical excision with a narrow clinical margin is first-line per NCCN; postoperative margin assessment confirms clearance.
• Electrodesiccation and curettage: acceptable for low-risk BCC and SCC in situ, but not for terminal hair–bearing skin (follicular extension escapes the curette) and not if fat is reached.
• Topical field therapy for superficial BCC, actinic keratosis, and Bowen disease: antimetabolite 5-fluorouracil (avoid in pregnancy and in known dihydropyrimidine dehydrogenase [DPD] deficiency), immune-response modifier imiquimod, photodynamic therapy, or cryotherapy. These are inadequate for nodular or invasive disease.

High-risk lesions (central face/"H-zone," ears, lips, recurrent, infiltrative/morpheaform, perineural invasion, immunosuppressed host)

• Mohs micrographic surgery: complete circumferential peripheral and deep margin assessment; endorsed by NCCN and by the AAD/ACMS/ASDSA/ASMS Appropriate Use Criteria as the modality of choice for these tumors.
• Adjuvant radiotherapy for extensive perineural or incompletely excised disease; primary radiotherapy for non-surgical candidates.

Locally advanced or metastatic disease

• BCC — hedgehog/smoothened inhibitors: vismodegib (locally advanced or metastatic) or sonidegib (locally advanced only), exploiting the PTCH1-loss mechanism.
• SCC — anti-PD-1 checkpoint inhibitor: cemiplimab (or pembrolizumab), reflecting the high mutational burden of UV-driven SCC.

Contraindicated / cautioned

• Hedgehog inhibitors in pregnancy: severe embryopathy; strict contraception required.
• Radiotherapy in Gorlin syndrome and xeroderma pigmentosum: provokes new tumors in the treated field.
• Checkpoint inhibitors in organ-transplant recipients: risk of allograft rejection; instead consider reducing immunosuppression or converting to an mTOR inhibitor (sirolimus), plus acitretin (teratogenic) for chemoprophylaxis.
• Oral nicotinamide: reduced new keratinocyte carcinomas in immunocompetent high-risk patients with prior skin cancers (ONTRAC); a randomized trial in solid-organ transplant recipients (ONTRANS) did not show benefit, so it is not a transplant-specific prophylactic measure.

Complications of the disease

• **Local destructive invasion (rodent ulcer)**: BCC grows along paths of least resistance into cartilage, bone, and orbit; the signal is a non-healing ulcer with a rolled border that recurs after every "scab." Periocular tumors threaten the globe.
• Perineural invasion: tumor tracks along nerve sheaths, most often trigeminal (V) or facial (VII) branches. The finding is numbness, paresthesia, burning pain, or new facial weakness over a tumor-bearing zone — mandates MRI and radiation oncology input, since clinical margins grossly underestimate extent.
• Regional nodal metastasis (SCC): predicted by poor differentiation, depth beyond fat, diameter, lip/ear location, and immunosuppression. Signal is a firm fixed node — parotid/cervical for ear and cheek primaries.
• Distant metastasis: uncommon; lung most often. Advanced SCC may secrete PTHrP, producing humoral hypercalcemia of malignancy (confusion, polyuria, short QT) — an emergency.
• Marjolin ulcer: SCC arising in a chronic burn scar, sinus tract, or non-healing wound; sudden change in a stable scar is the tip-off. Aggressive, higher metastatic rate.
• Catastrophic hemorrhage: neglected scalp/neck tumors eroding a named vessel — an emergency requiring pressure, resuscitation, and surgical control.
• Field cancerization: ~40% develop a second NMSC within 5 years; justifies lifelong total-body skin examination as recommended by NCCN.

Complications of treatment

• Surgical: scarring, ectropion or eyelid retraction after periocular excision, motor nerve injury, and recurrence — highest with infiltrative/morpheaform subtypes cleared by non-margin-controlled techniques.
• Radiotherapy: chronic radiation dermatitis, ulceration, and radiation-induced second cutaneous malignancy after a long latency.
• Hedgehog inhibitors: muscle spasms, dysgeusia, alopecia, weight loss; teratogenicity is absolute.
• Checkpoint inhibitors: immune-related adverse events — colitis, hepatitis, thyroiditis/hypophysitis, pneumonitis, and myocarditis, the last a true emergency presenting with troponin elevation and conduction block; treat with high-dose corticosteroids.
• Topical 5-FU/imiquimod: intense inflammatory erosion of the treated field, expected but often mistaken for infection.

• Histology pair to memorize: peripheral palisading of basaloid nuclei with retraction (clefting) artifact in a myxoid stroma = BCC; keratin pearls plus intercellular bridges = SCC. Keratinization essentially excludes BCC.
• Single best next step for any suspicious, non-healing, bleeding lesion: biopsy (shave or punch). Cryotherapy, 5-FU, or excision before histology is the classic wrong answer, because subtype and depth determine whether Mohs is required.
• The association examiners love: Gorlin (nevoid BCC) syndrome — autosomal dominant PTCH1 loss with unopposed hedgehog signaling → multiple early BCCs, odontogenic keratocysts, palmar/plantar pits, calcified falx cerebri, and medulloblastoma. Avoid radiotherapy in these patients.
• Immunohistochemistry tiebreaker: BerEP4- and MOC-31-positive, EMA-negative favors BCC (both BerEP4 and MOC-31 target EpCAM); EMA-positive, BerEP4/MOC-31-negative with strong high-molecular-weight cytokeratin (AE1/AE3) and p63 staining favors SCC. Note that the article's Diagnosis section lists EMA among BCC-positive markers — the conventional teaching is the reverse: EMA favors squamous differentiation.
• Transplant recipients flip the ratio: in the general population BCC outnumbers SCC roughly 4:1, but under chronic immunosuppression SCC predominates and behaves aggressively — a stem describing a kidney transplant patient with a fast-growing scalp nodule is pointing at SCC.
• Site matters more than size on the exam: central face "H-zone," eyelids, ears, and lips are high-risk locations → Mohs micrographic surgery (AAD/ACMS Appropriate Use Criteria), not simple excision or curettage.
• Common distractor — metastasis: BCC is the most common human cancer yet almost never metastasizes; its morbidity is local destruction. Do not order staging CT for a nodular facial BCC. Conversely, SCC of the lower lip or ear is the one you stage clinically for nodes.
• Precursor logic: actinic keratosis is SCC in situ's dysplastic precursor and warrants field therapy; keratoacanthoma — rapid dome-shaped nodule with a central keratin plug that may involute — is treated as a well-differentiated SCC variant, not observed.