Keratinocytic pathology (BCC, SCC, actinic keratosis)

Meet the patient — two pots on the pathologist's tray

Two specimens arrive together. The first is a punch from a gritty, scaly patch on a farmer's lower lip — a lesion the clinician suspects is more than the actinic keratosis it resembles. The second is a shave from a pearly, translucent nodule on an elderly nose. The pathologist's job is identical for both: decide how thick the atypia is, whether the basement membrane is breached, and what subtype sits under the lens — because those three answers change the surgery that follows.[1][4]

One principle frames the whole leaf: partial, full, or through. Partial-thickness atypia is AK; full-thickness atypia still boxed above the basement membrane is Bowen; atypia through the membrane into the dermis is invasive SCC. Hold that ladder and the rest of the report writes itself.[3][4]

The ladder — partial, full, through

The keratinocytic spectrum climbs in three rungs, and the rung is set by how far the atypical keratinocyte has climbed. Memorise the mapping, because it is the single most tested fact in this leaf.[1][6]

[1]

The borders are continuous in real life. When AK atypia becomes full-thickness, the language shifts to SCC in situ; when Bowen breaks through, it is invasive SCC. Clinicopathologic correlation prevents false comfort at those transitions — a biopsy that reads "AK" does not clear the field around it.[7][8]

Field cancerization — the scaly papule is the tip of the iceberg

Chronic ultraviolet exposure does not produce one mutant clone; it produces a field of genetically altered keratinocytes across the chronically sun-damaged skin. The gritty papule you freeze is the visible lesion; the surrounding epidermis carries subclinical dysplasia that explains why new lesions keep appearing next to treated ones.[7]

This is why AAD guidance frames AK management as both lesion-directed (cryotherapy of individual lesions) and field-directed (5-FU, imiquimod, photodynamic therapy) when the burden is high, alongside lifelong photoprotection. A single banal-looking AK biopsy does not clear the field — counsel the patient accordingly.[8][9]

Sampling that makes the pathology useful — or useless

Technique decides whether subtype and invasion can be assessed at all. The commonest cause of an uninterpretable report is a biopsy that is too superficial or too small for the question being asked.[3][5]

[1]

Always state on the request form: site, size, clinical diagnosis and differential, prior treatment (cryotherapy, 5-FU, imiquimod, photodynamic therapy), immunosuppression, and whether margins are diagnostic only. The pathologist can only report what the pot allows.[1]

Invasive SCC — the report elements that change surgery

Once the membrane is breached, the report stops being descriptive and becomes a surgical plan. Five elements decide margin strategy, nodal consideration, and whether the case belongs in an MDT.[1][4]

• Depth, thickness, and anatomic level — the primary local risk driver; deeper invasion raises recurrence and metastasis.[3][5]
• Differentiation — well, moderate, or poor; poor differentiation marks an aggressive lesion.[1]
• Perineural invasion (PNI) — recurrence and an aggressive pathway; flags referral for wider margins and possible radiotherapy.[4]
• Lymphovascular invasion — sets metastatic context and nodal surveillance.[6]
• Margins, peripheral and deep — the immediate re-excision decision; never leave a positive deep margin unaddressed.[1]

[1]

Host context is part of the risk. A transplant recipient, a patient with chronic lymphocytic leukaemia, or one on long-term immunosuppression carries a markedly higher SCC risk and worse outcomes — the report should prompt the clinician to act on the host, not just the lesion.[5][6]

Keratoacanthoma — the crater that may not be benign

The keratoacanthoma (KA) presents clinically as a rapidly growing crateriform nodule filled with keratin, and histologically as a well-differentiated crateriform keratinocytic proliferation that can closely mimic well-differentiated SCC.[12]

The board stance: many centres now manage KA-like lesions with complete excision and pathological review rather than assuming the textbook spontaneous resolution — especially in adults and immunosuppressed patients, where the risk of misclassifying an aggressive SCC as a self-limiting KA is highest. The histology alone often cannot settle KA from well-differentiated SCC, which is why a complete specimen and clinical correlation carry the decision.[12][4]

Basal cell carcinoma — subtype is destiny

BCC is the commonest keratinocyte carcinoma clinically, and European interdisciplinary guidance and clinical reviews stress that diagnosis, subtype, and treatment selection are inseparable. The subtype on the report is not a footnote; it is the instruction that chooses Mohs over a standard excision.[1][2]

Classic nodular BCC histology is the archetype to memorise: basaloid nests of varying size, peripheral palisading of nuclei at the nest edge, retraction artefact (the stromal clefting that separates tumour from stroma), and a mucinous stroma in many cases.[1]

[1]

Immunohistochemistry — support, not magic

Ber-EP4 classically labels BCC and is the stain that separates BCC from SCC when the nests are ambiguous — a use established by Tellechea in 1993 and still taught.[10] But the limitation that fails candidates is the one to remember: Bowen disease can show Ber-EP4 reactivity, so a brown stain at the in-situ-carcinoma interface is not proof of BCC. Morphology and the full panel context still rule.[11]

EMA, high-molecular-weight keratins, and p63 or p40 enter selected adnexal and spindle differentials — but never as a lone "SCC stain" applied without architecture. The cardinal rule of dermatopathology holds here: stains support the morphology; they do not replace it.[1][11]

Differentials that catch candidates

The histological mimics cluster around two traps — a reactive proliferation read as SCC, and a benign adnexal tumour read as BCC. The mitigations are pattern recognition and, where needed, a panel.[1]

[1]

From report to management — the board reading habit

A disciplined pathologist reads a keratinocytic specimen in a fixed order, and a candidate should be able to reproduce that order at the viva.[1]

[1]

The mapping to management follows that reading, with principles rather than a dose table:[1]

• AK — lesion- and field-directed options per AAD guidance; photoprotection always.[8][9]
• SCC in situ — complete destruction or excision with clinical margin discipline; never leave untreated full-thickness atypia on a critical site.[4]
• Invasive SCC — risk-adapted surgical margins; high-risk features escalate to specialist and MDT.[3][4][5]
• BCC — low-risk nodular and superficial pathways versus high-risk infiltrative and micronodular subtypes, which demand Mohs or careful margin control per the European consensus framing.[1][2]

Regional and exam notes

The verticals ask different things of the same spectrum. MBBS and NEET-PG expect the spectrum table and the red flags; MRCP and IADVL add transplant SCC risk and field cancerization; FACD, FRCDerm, and ABD push subtype language, PNI reporting, Ber-EP4 limits, and Mohs indications. In skin of colour, pigmented BCC and under-recognised AK on non-classic sites demand a deliberate examination — the pathology is the same, but the clinical suspicion is harder to earn.[1][5]

The mantra, and the memory device

[1]

The mantra: architecture and invasion first; subtype second; stains support, never supplant.[1]

Etymology for viva gold: Bowen disease honours John T. Bowen, the American dermatologist who described it in 1912; basal cell carcinoma is named for the basaloid cells that mimic the epidermal basal layer from which they derive. Keratoacanthoma stitches Greek keratos (horn) plus akantha (thorn) plus -oma — a horned thorn of a tumour, which is exactly its crateriform silhouette.[1][12]

Ward-round test — three stems, thirty seconds each

Stem 1 — the superficial shave that 'rules out SCC' (answer)

A registrar shaves the surface of a thick, gritty lower-lip plaque in a transplant recipient and the report reads 'actinic keratosis, base transected'. The lesion is clinically indurated and fixed. Is SCC excluded? Model: No. A superficial shave with a transected base cannot exclude invasive SCC — it has sampled only the top of a clinically aggressive tumour and understaged it. The right move is a full-thickness punch or incisional biopsy to deep dermis, or definitive excision with orientation. Treat a transected base on a thick keratinocytic tumour as incomplete staging information, not reassurance, especially in an immunosuppressed host whose SCC behaves aggressively.[1][5]

Stem 2 — a Ber-EP4-positive full-thickness atypical epidermis (answer)

A scaly plaque on the temple biopsies as full-thickness keratinocyte atypia, but Ber-EP4 stains positive in the atypical cells. The registrar calls it basal cell carcinoma. What is the error? Model: The error is diagnosing BCC from a single stain. Bowen disease (SCC in situ) is well documented to show Ber-EP4 reactivity, so a positive stain at the in-situ interface does not prove BCC. Architecture decides: full-thickness atypia with an intact basement membrane is SCC in situ (Bowen), managed by complete destruction or excision — not a BCC pathway. Ber-EP4 supports morphology; it never supplants it.[10][11]

Stem 3 — a pearly nose nodule read as 'BCC, infiltrative strands' (answer)

A shave from a pearly nasal nodule reports basal cell carcinoma with infiltrative strands in a sclerotic stroma, deep margin positive. The registrar plans a standard 4 mm excision. What is wrong? Model: Infiltrative or morpheaform BCC has ill-defined clinical borders and the highest recurrence, so a standard excision risks a positive margin and recurrence. This subtype belongs on the Mohs or careful margin-control pathway, and the positive deep margin mandates re-excision regardless. Subtype is destiny in BCC — report it, and let it choose the surgery.[1][2]

References

1. [1]Peris K, Fargnoli MC, Kaufmann R, et al. European consensus-based interdisciplinary guideline for diagnosis and treatment of basal cell carcinoma-update 2023 Eur J Cancer, 2023.PMID 37604067
2. [2]Heath MS, Bar A. Basal Cell Carcinoma Dermatol Clin, 2023.PMID 36410973
3. [3]Waldman A, Schmults C. Cutaneous Squamous Cell Carcinoma Hematol Oncol Clin North Am, 2019.PMID 30497667
4. [4]Wysong A. Squamous-Cell Carcinoma of the Skin N Engl J Med, 2023.PMID 37314707
5. [5]Que SKT, Zwald FO, Schmults CD. Cutaneous squamous cell carcinoma: Incidence, risk factors, diagnosis, and staging J Am Acad Dermatol, 2018.PMID 29332704
6. [6]Jiang R, Fritz M, Que SKT. Cutaneous Squamous Cell Carcinoma: An Updated Review Cancers (Basel), 2024.PMID 38791879
7. [7]Willenbrink TJ, Ruiz ES, Cornejo CM, et al. Field cancerization: Definition, epidemiology, risk factors, and outcomes J Am Acad Dermatol, 2020.PMID 32387665
8. [8]Eisen DB, Asgari MM, Bennett DD, et al. Guidelines of care for the management of actinic keratosis: Executive summary J Am Acad Dermatol, 2021.PMID 34111497
9. [9]Eisen DB, Asgari MM, Bennett DD, et al. Guidelines of care for the management of actinic keratosis J Am Acad Dermatol, 2021.PMID 33820677
10. [10]Tellechea O, Reis JP, Domingues JC, et al. Monoclonal antibody Ber EP4 distinguishes basal-cell carcinoma from squamous-cell carcinoma of the skin Am J Dermatopathol, 1993.PMID 8238781
11. [11]Kogut M, Toberer F, Enk AH, et al. Limitations of Ber-EP4 for distinction of Bowen disease from basal cell carcinoma J Cutan Pathol, 2016.PMID 26765054
12. [12]Bahmad HF, Stoyanov K, Mendez T, et al. Keratoacanthoma versus Squamous-Cell Carcinoma: Histopathological Features and Molecular Markers Dermatopathology (Basel), 2024.PMID 39449378