Squamous cell carcinoma (SCC) is the “workhorse” skin cancer on USMLE—common, very testable, and packed with classic associations (sun exposure, immunosuppression, actinic keratosis). If you can quickly recognize its risk factors, typical lesions, and the few situations where it behaves aggressively, you’ll pick up easy points on both Step 1 (path + associations) and Step 2 (management).
Big Picture (What SCC is)
Squamous cell carcinoma is a malignant tumor of keratinocytes arising from the epidermis (often from actinic keratosis or in chronically injured/inflamed skin). It is the second most common skin cancer after basal cell carcinoma and has a real metastatic potential (higher than BCC).
High-yield one-liner:
SCC = malignant keratinocytes → scaly/ulcerated erythematous plaque or nodule on sun-exposed skin; can metastasize, especially on lip/ear or in immunosuppressed patients.
First Aid Cross-References (where this shows up)
You’ll see SCC content split across:
- Dermatology: Skin cancer (BCC vs SCC vs melanoma)
- Pathoma/FA: Neoplasia basics + UV-related DNA damage
- Micro/Immuno tie-ins: HPV, immunosuppression (transplant meds), HIV
First Aid hooks to remember
- SCC: “Scaly, red, ulcerated lesion that may bleed; can metastasize.”
- Actinic keratosis as a premalignant lesion (often grouped right next to SCC).
Pathophysiology (Step 1 core)
UV radiation → DNA damage
- UVB causes pyrimidine (thymine) dimers
- Normally fixed by nucleotide excision repair
- With repeated injury or repair failure → mutations accumulate in keratinocytes
Tumor suppressor genes
- p53 mutations are classic in UV-associated skin cancers
- p53 normally halts the cell cycle for DNA repair or triggers apoptosis
Histology: what you’re expected to recognize
- Atypical squamous cells invading dermis
- Keratin pearls (concentric keratinization) = classic buzzword
- Intercellular bridges may be noted
| Feature | High-yield point |
|---|---|
| Origin | Keratinocytes (epidermis) |
| UV association | Strong (especially cumulative sun exposure) |
| Histology | Keratin pearls, atypical squamous cells |
| Behavior | Can metastasize (more than BCC) |
Risk Factors & High-Yield Associations
Sun exposure (cumulative) + fair skin
- Think: older patients, outdoor occupations, “weathered” skin
Premalignant lesion: Actinic keratosis
- Rough, scaly papules/plaques on sun-exposed skin
- Considered premalignant → may progress to SCC
Immunosuppression (very testable)
- Solid organ transplant patients on chronic immunosuppressants have a markedly increased SCC risk (often multiple, more aggressive)
HPV association (esp. mucosal/anogenital)
- HPV 16/18 classically associated with SCC of cervix/anogenital region
- Cutaneous HPV types can contribute to some skin SCCs (esp. in certain settings)
Chronic inflammation/injury → SCC
Classic “SCC in scars” association:
- Marjolin ulcer = SCC arising in chronic wounds/burn scars
- Often more aggressive than typical sun-induced SCC
Arsenic exposure
- Can predispose to SCC (plus other skin findings); less common but board-relevant
Where it’s more likely to metastasize
- Lower lip and ear lesions are higher risk
- Also: larger/deeper tumors, perineural invasion, immunosuppression
Clinical Presentation (how it appears on vignettes)
Typical SCC descriptions:
- Firm, erythematous, scaly papule/plaque or nodule
- May become ulcerated and bleed
- Often on sun-exposed areas: face, scalp, neck, dorsum of hands, forearms
Classic contrast you should know
- BCC: pearly papule with rolled borders + telangiectasias; locally invasive, rare mets
- SCC: scaly/ulcerated, indurated lesion; metastatic potential
- Melanoma: ABCDE; aggressive mets
Diagnosis (Step 1 + Step 2 workflow)
Initial evaluation
- Full skin exam + risk factor assessment
- Pay attention to:
- Size, depth/induration
- Location (lip/ear high risk)
- Immunosuppression
- Rapid growth, ulceration, pain (possible perineural invasion)
Biopsy is the key step
- Shave biopsy often used for superficial lesions (varies by lesion/location)
- Punch biopsy can be used when depth is needed
- Excisional biopsy sometimes preferred if melanoma is in the differential (don’t shave a suspected melanoma on exams)
Histologic confirmation: keratinization/keratin pearls + invasive atypical squamous cells.
Treatment (Step 2 high yield, but Step 1-relevant)
Treatment depends on risk category (low vs high) and location.
Surgical
- Excision with appropriate margins: common approach for low-risk SCC
- Mohs micrographic surgery:
- Tissue-sparing, high cure rate
- Preferred for high-risk tumors or cosmetically/functionally sensitive areas (face, ears, lips), recurrent tumors, ill-defined borders
Nonsurgical options (selected cases)
- Curettage and electrodesiccation: small, low-risk lesions
- Topical 5-fluorouracil (5-FU) or imiquimod:
- More for actinic keratoses or SCC in situ (Bowen disease) rather than invasive SCC
- Radiation therapy:
- For non-surgical candidates or certain high-risk situations
Advanced/metastatic disease
- Regional lymph node evaluation if clinically indicated (high-risk lesions, palpable nodes)
- Systemic therapy options exist (e.g., immunotherapy) but typically beyond Step 1 depth; Step 2 may mention for advanced cases.
SCC Variants You Should Know
SCC in situ (Bowen disease)
- Malignant keratinocytes confined to epidermis (no dermal invasion yet)
- Appears as well-demarcated scaly erythematous plaque
- Treated with topical therapy, excision, or destructive modalities depending on case
Keratoacanthoma (classic trap)
- Rapidly growing crateriform nodule with keratin plug; can resemble SCC
- Often treated like SCC due to overlap and occasional aggressive behavior
High-Yield “Buzzwords” & Board Triggers
If you see this… think SCC:
- “Scaly,” “erythematous plaque,” “ulcerated,” “bleeds with minor trauma”
- “Sun-exposed” + “older” + “rough lesion”
- “Keratin pearls”
- “Transplant patient” with multiple aggressive skin lesions
- “Chronic burn scar turns into ulcer” → Marjolin ulcer (SCC)
Rapid Review Table (SCC vs BCC vs Melanoma)
| Feature | SCC | BCC | Melanoma |
|---|---|---|---|
| Cell of origin | Keratinocytes | Basal cells | Melanocytes |
| Typical look | Scaly, erythematous, indurated; may ulcerate | Pearly papule, rolled border, telangiectasias | Pigmented lesion with ABCDE |
| UV relationship | Cumulative exposure | UV exposure | Intermittent intense exposure; sunburns |
| Metastasis | Possible | Rare | Common (aggressive) |
| Histology buzzword | Keratin pearls | Palisading nuclei | Atypical melanocytes; Breslow depth matters |
USMLE-Style Mini Checkpoints
- Why does UV cause cancer? UVB → thymine dimers → p53 mutation risk increases.
- What precursor lesion leads to SCC? Actinic keratosis (rough, scaly sun-exposed lesions).
- Which skin cancer metastasizes more: BCC or SCC? SCC.
- Which locations are higher risk for SCC metastasis? Lower lip and ear (also immunosuppressed patients).
- SCC in a burn scar is called? Marjolin ulcer.
Takeaway (what to remember on test day)
If the stem screams sun damage + scaly/ulcerated lesion, you should reflexively think SCC, confirm with biopsy, and treat with excision or Mohs depending on risk. The “extra points” come from knowing it metastasizes, especially in immunosuppressed patients and high-risk sites like the lip/ear, and from recognizing keratin pearls and actinic keratosis as the setup.