Basal Cell Carcinoma

The most common cancer worldwide — UV-driven Hedgehog pathway activation, Gorlin syndrome, Mohs micrographic surgery, and vismodegib for advanced disease

Key Points

• Basal cell carcinoma (BCC) is the most common cancer in the world, with over 3.6 million cases diagnosed annually in the US alone.
• BCC almost never metastasizes (<0.1%), but can cause substantial local destruction — particularly on the face — if neglected.
• Ultraviolet radiation is the dominant cause; PTCH1 (Patched-1) inactivation and constitutive Hedgehog signaling drive pathogenesis in virtually all BCCs.
• Gorlin syndrome (nevoid BCC syndrome) causes multiple BCCs at a young age from germline PTCH1 mutations, along with odontogenic keratocysts and medulloblastoma.
• Mohs micrographic surgery is the gold standard for high-risk or facial BCC, achieving cure rates >99% with maximal tissue conservation.
• Vismodegib and sonidegib (Hedgehog pathway inhibitors) are FDA approved for locally advanced or metastatic BCC; cemiplimab is approved after HHI failure.

Epidemiology & Incidence

Basal cell carcinoma is the most common malignancy in humans. In the United States, over 3.6 million BCCs are diagnosed annually in more than 2 million patients — more than all other cancers combined. Despite its overwhelming prevalence, BCC-specific mortality is extremely low (<0.1% metastasize), and the vast majority are cured with local treatment. However, BCC can cause profound local tissue destruction, disfigurement, and functional impairment when neglected, particularly on the face. BCC arises from the basal cells of the epidermis and its appendages. It does not arise from the dermis…

Molecular Biology & Risk Factors

The Hedgehog (HH) signaling pathway is the central oncogenic driver in virtually all BCCs: • Under normal conditions: PTCH1 (Patched-1 receptor) suppresses SMO (Smoothened), preventing Hedgehog target gene activation. • In BCC: UV-induced mutation of PTCH1 → loss of SMO suppression → constitutive activation of downstream GLI transcription factors (GLI1, GLI2) → uncontrolled cell proliferation. • PTCH1 mutations are found in >85% of sporadic BCCs; SMO activating mutations in ~10%. • UV signature mutations (C→T transitions at dipyrimidine sites) are the predominant mutational pattern,…

Clinical Presentation & Subtypes

BCC most commonly occurs on sun-exposed areas of the head and neck (~80%), particularly the nose, eyelids, forehead, and cheeks. Trunk (shoulders, back) accounts for ~15%; extremities ~5%. Histologic/clinical subtypes: Nodular BCC (most common, ~60%): • Pearly, translucent papule or nodule with rolled ("pearly") borders and visible telangiectasias. • Central ulceration common in larger lesions ("rodent ulcer" — a historical term). • Classic appearance: Shiny, skin-colored or pink, well-demarcated; bleeds easily with minor trauma. Superficial BCC (~30%): • Erythematous, scaly, thin plaque…

Treatment

The choice of treatment depends on tumor subtype, location, size, patient age/health, and prior treatment history: Surgical treatment (primary approach): • Mohs Micrographic Surgery (MMS): Gold standard for high-risk BCC — facial/cosmetically sensitive sites (nose, eyelids, ears, lips, temples), large tumors (>2 cm), poorly defined margins, recurrent BCC, morpheaform/infiltrating histology, or perineural/perivascular invasion. Staged excision with complete margin examination (100% of margins) achieves 5-year cure rates >99% for primary BCC, ~95% for recurrent BCC. Allows maximum tissue…