ABPath Dermatopathology cheat sheet

Psoriasis: regular acanthosis with club-shaped rete ridges, thinned suprapapillary plates, parakeratosis with neutrophils (Munro microabscesses), and dilated papillary dermal capillaries.

Lichen planus: wedge-shaped hyperkeratosis, band-like lymphocytic infiltrate obscuring the dermoepidermal junction, basal vacuolar change, and Civatte bodies (apoptotic keratinocytes).

Pemphigus vulgaris shows suprabasal acantholysis with a tombstone row of basal cells; direct immunofluorescence reveals intercellular IgG in a net-like epidermal pattern.

Bullous pemphigoid shows subepidermal blister with eosinophils; direct immunofluorescence reveals linear IgG and C3 along the basement membrane zone.

Dermatitis herpetiformis: subepidermal blisters with neutrophilic papillary microabscesses; direct immunofluorescence shows granular IgA in dermal papillae.

Mycosis fungoides: epidermotropic atypical lymphocytes with cerebriform nuclei, Pautrier microabscesses, and a band-like upper dermal infiltrate; CD4-positive T cells predominate.

Lupus erythematosus: interface dermatitis with vacuolar change, mucin in the dermis, and perivascular/periadnexal lymphocytic infiltrate; direct immunofluorescence shows granular IgG/C3 at the basement membrane (lupus band).

Melanoma key prognostic features: Breslow thickness (mm), ulceration, mitotic rate, and microsatellitosis; sentinel lymph node status informs staging.

Kaposi sarcoma: slit-like vascular spaces with extravasated erythrocytes, spindle cells, and hyaline globules; HHV-8 immunohistochemistry is positive.

ABCDE criteria for melanoma: Asymmetry, Border irregularity, Color variation, Diameter greater than 6 mm, Evolution (change over time).

Basal cell carcinoma: pearly papule with telangiectasia and rolled border; histology shows peripheral palisading, retraction artifact, and mucin.

Squamous cell carcinoma: hyperkeratotic or ulcerated lesion; histology shows atypical keratinocytes invading the dermis with keratin pearls.

Actinic keratosis: rough, scaly macule on sun-exposed skin; histology shows atypical keratinocytes confined to the lower epidermis (partial-thickness atypia).

Seborrheic keratosis: stuck-on waxy plaque; histology shows horn pseudocysts and basaloid cells with a flat base.

Dermoscopy of pigmented lesions: pigment network (melanocytic), blue-white veil, irregular dots/globules, and atypical vascular patterns suggest malignancy.

Fluorescence microscopy uses fluorophore-conjugated antibodies (e.g., FITC) to detect immune deposits or organisms; direct immunofluorescence is performed on perilesional skin.

Polarized vs nonpolarized dermoscopy: polarized light visualizes deeper structures and vascular patterns; nonpolarized light requires liquid interface and highlights surface keratin.

Punch biopsy (3-4 mm) is standard for inflammatory dermatoses; excisional biopsy is preferred for suspected melanoma to allow Breslow measurement.

Cytokeratin (AE1/AE3, CK5/6) marks epithelial cells; S100, SOX10, Melan-A, and HMB-45 mark melanocytes; CD45 marks leukocytes.

CD3 and CD20 distinguish T-cell from B-cell infiltrates; CD30 marks large activated lymphocytes (e.g., lymphomatoid papulosis, anaplastic large cell lymphoma).

CD34 highlights vascular endothelium and dermal dendrocytes; D2-40 (podoplanin) marks lymphatic endothelium.

Desmin and smooth muscle actin mark smooth muscle; CD68 marks histiocytes; CD1a and langerin mark Langerhans cells.

Direct immunofluorescence panels for blistering disease: IgG, IgA, IgM, C3, and fibrinogen on perilesional skin.

Periodic acid-Schiff (PAS) highlights fungal walls and basement membrane; Fontana-Masson highlights melanin; Prussian blue highlights iron.

T-cell receptor gene rearrangement and immunoglobulin gene rearrangement studies assess clonality in suspected lymphoma.

BRAF V600E mutation testing guides targeted therapy in melanoma; sentinel node and molecular staging inform prognosis.

In situ hybridization for EBV (EBER) and HHV-8 supports diagnosis of associated lymphomas and Kaposi sarcoma.

Specimen labeling requires two patient identifiers (name and date of birth or medical record number) plus site and laterality; mislabeled specimens are rejected.

Formalin fixation: 10% neutral buffered formalin at approximately 10:1 fixative-to-tissue ratio; overfixation can impair immunohistochemistry.

Turnaround time metrics track accession to sign-out; quality indicators include amended reports, frozen section concordance, and diagnostic error rates.

Frozen section is used for intraoperative margin assessment; it has lower diagnostic accuracy than permanent sections and is not definitive for all diagnoses.

Proficiency testing and peer review (e.g., interobserver concordance studies) are components of laboratory quality assurance.

Critical value and addendum reporting policies ensure timely communication of significant or amended diagnoses to the treating clinician.

CLIA and CAP accreditation standards govern personnel qualifications, instrument maintenance, and quality control documentation.

When histology and clinical impression conflict, clinicopathologic correlation and repeat or deeper biopsy are indicated before definitive therapy.

Consultation requests should include clinical history, lesion site and duration, prior biopsies, and specific diagnostic questions.

Second-opinion review compares original and referral slides; discordance should be documented with reasons and communicated to the referring pathologist.

When a diagnosis remains uncertain, a descriptive diagnosis with a differential and recommended ancillary tests or rebiopsy is appropriate.

Immunohistochemistry and molecular testing should be ordered selectively to resolve specific differential diagnoses rather than as broad panels.

Communication of a changed diagnosis (amendment) requires documentation and direct notification of the responsible clinician.

Multidisciplinary tumor board review is indicated for melanoma and complex cutaneous lymphomas to integrate pathology, staging, and treatment.

Consultation reports should state the final diagnosis, supporting features, differential considerations, and any limitations of the material reviewed.

Interface dermatitis differential includes lichen planus, lupus erythematosus, erythema multiforme, and drug eruption; pattern and depth of infiltrate help distinguish them.

Spongiotic dermatitis (eczema) shows intercellular edema with eosinophils in allergic contact and variable acanthosis; chronic lesions show hyperkeratosis and lichenification.

Granulomatous dermatitis differential includes sarcoidosis (naked noncaseating granulomas), infection (caseating), and foreign body reaction.

Vasculitis: fibrinoid necrosis of vessel walls with neutrophilic infiltrate and leukocytoclasia; direct immunofluorescence may show immune complexes.

Panniculitis: septal (erythema nodosum) versus lobular (erythema induratum) patterns guide differential diagnosis.

Pigmented lesions: pagetoid spread, poor circumscription, and dermal mitoses favor melanoma over benign nevi.

Alopecia biopsy patterns: nonscarring (lichen planopilaris is scarring) versus scarring alopecia with loss of follicular units and fibrosis.

H&E staining: hematoxylin stains nuclei blue-purple (basophilic); eosin stains cytoplasm and collagen pink (eosinophilic).

Section thickness for routine light microscopy is typically 4-5 micrometers; thicker sections may be needed for some special stains.

Orientation of specimens (e.g., shave, punch, excision) affects assessment of margins and depth; Mohs sections are frozen and horizontally oriented.

Polarization microscopy identifies foreign material (suture, talc) and collagen birefringence.

Immunofluorescence requires fresh or Michel's medium-transported tissue; formalin fixation destroys many immune deposits.

Microscopy magnification conventions: low power (2x-4x) for architecture, high power (40x) for cytology; oil immersion (100x) for fine nuclear detail.

Melanoma versus carcinoma: SOX10 and Melan-A support melanoma; cytokeratins support carcinoma; p63/p40 support squamous differentiation.

Merkel cell carcinoma: cytokeratin 20 dot-like positivity, neuroendocrine markers (synaptophysin, chromogranin), and Merkel cell polyomavirus testing.

Basal cell carcinoma: Ber-EP4 positive; squamous cell carcinoma is typically Ber-EP4 negative.

Dermatofibrosarcoma protuberans: CD34 positive with COL1A1-PDGFB fusion; helpful for margin assessment.

Sebaceous carcinoma: EMA and androgen receptor positive; adipophilin highlights lipid droplets.

Lymphoma workup panels include CD3, CD4, CD8, CD20, CD30, and CD56, with clonality studies as indicated.

Molecular testing in melanoma includes BRAF, NRAS, and KIT mutations to guide targeted therapy selection.

Quality control for special stains includes positive and negative control slides run with each batch.

Specimen tracking from accession through sign-out prevents lost or misidentified cases; barcode systems reduce transcription errors.

Slide and block retention policies follow regulatory requirements; blocks are retained for years to permit re-review.

Turnaround time and error rates are monitored as quality indicators; root cause analysis follows significant errors.

Documentation of instrument maintenance and reagent lot numbers supports traceability during investigations.

Delegation of grossing and frozen section duties requires documented competency assessment under laboratory policy.

Referral material should be returned with the consultation report; slides and blocks remain the property of the originating laboratory per agreement.

The consultation report should specify material reviewed, ancillary studies performed, final diagnosis, and any recommendations.

Direct verbal communication of urgent or unexpected diagnoses should be documented with date, time, and person notified.

When consultation and original diagnosis differ, the referring pathologist should be informed to allow reconciliation and patient communication.

Expert consultation is appropriate when the diagnosis would materially change management, such as melanoma versus atypical nevus or lymphoma subtyping.

Psoriasiform dermatitis differential includes psoriasis, pityriasis rubra pilaris, and chronic spongiotic dermatitis; check for parakeratosis and neutrophil microabscesses.

Lichenoid dermatitis differential includes lichen planus, lichenoid drug eruption, and lupus; drug eruptions often show eosinophils and parakeratosis.

Sweet syndrome: neutrophilic dermal infiltrate without vasculitis; histiocytoid variant can mimic leukemia cutis.

Erythema multiforme: interface dermatitis with necrotic keratinocytes and a sparse infiltrate; target lesions clinically.

Morphea: thickened collagen bundles with loss of adnexal structures and a sparse lymphocytic infiltrate.

Granuloma annulare: palisading granulomas around mucin in the dermis; distinguish from necrobiosis lipoidica (layered collagen degeneration).

Lesion description should include size, shape, color, border, surface, and anatomic site with laterality.

Excisional biopsy with 2-3 mm margins is preferred for suspected melanoma; shave biopsy may preclude accurate Breslow measurement.

Punch biopsy is appropriate for inflammatory dermatoses and deep dermal processes; a 4 mm punch is commonly used.

Incisional biopsy is used for large tumors where complete excision is impractical; sampling should target the most representative area.

Specimen orientation with sutures or ink aids margin assessment in excisions.

Photographic documentation of gross lesions supports correlation with histology and follow-up.

Immunohistochemical staining interpretation requires attention to internal controls and background; weak or absent staining may reflect fixation artifact.

Aberrant cytokeratin expression can occur in sarcomas and lymphomas; panels rather than single markers improve specificity.

Sentinel lymph node evaluation uses H&E plus immunohistochemistry (S100, Melan-A, HMB-45) to detect micrometastases.

Clonality testing can yield false-positive results in reactive inflammatory infiltrates; interpret in morphologic context.

Fungal stains (PAS, GMS) are more sensitive than H&E; cultures may be needed for species identification and susceptibility.

Margin status in melanoma and squamous cell carcinoma guides re-excision decisions; margin distance requirements vary by tumor type and stage.

A diagnosis of melanoma triggers staging considerations including sentinel lymph node biopsy for appropriate thickness.

Unexpected malignant diagnoses require timely communication with the clinical team to coordinate further management.

Correlation with clinical and dermoscopic findings reduces diagnostic error in pigmented lesions.

Biopsy of a suspected drug eruption should be timed to capture characteristic histologic features and include perilesional skin for immunofluorescence when blistering disease is suspected.

Requests for second opinion should specify whether the consultation is for diagnosis only or for management recommendations.

Receipt and return of slides should be logged to maintain chain of custody and specimen tracking.

Consultation turnaround expectations should be communicated; urgent cases (e.g., suspected lymphoma) may require expedited review.

Disagreement between consultants should prompt review of additional levels, ancillary studies, or rebiopsy rather than arbitrary selection of one opinion.

Consultation fees and billing documentation should reflect the material reviewed and services performed.