ABD Dermatopathology cheat sheet

Interface dermatitis with vacuolar change and a bandlike lymphocytic infiltrate is characteristic of lichen planus; wedge-shaped hyperkeratosis, saw-tooth rete ridges, and PAS-positive basement membrane thickening support the diagnosis.

Spongiotic dermatitis shows intercellular edema; eosinophils favor allergic contact dermatitis or drug eruption, while neutrophils in the stratum corneum suggest impetiginization or an irritant reaction.

Psoriasiform dermatitis displays regular acanthosis, thinned suprapapillary plates, dilated capillaries, and Munro microabscesses; the granular layer is often reduced.

Pemphigus vulgaris direct immunofluorescence shows intercellular IgG and C3 in a chicken-wire pattern; bullous pemphigoid shows linear IgG and C3 along the basement membrane zone.

Dermatitis herpetiformis shows granular IgA in dermal papillae; linear IgA bullous dermatosis shows linear IgA along the basement membrane zone.

Leukocytoclastic vasculitis features fibrinoid necrosis of small vessel walls, neutrophilic dust, and extravasated red cells; direct immunofluorescence may show granular IgA in Henoch-Schonlein purpura.

Sarcoidosis produces naked noncaseating granulomas with sparse lymphocytic rims; tuberculoid leprosy often shows perineural granulomatous inflammation.

Sweet syndrome shows a dense neutrophilic dermal infiltrate without true vasculitis; pyoderma gangrenosum shows ulceration with an undermined edge and a mixed neutrophilic infiltrate.

Benign nevi are symmetric, well circumscribed, and show maturation: nevus cells become smaller and less nested with increasing dermal depth, and dermal mitoses are absent.

Melanoma shows asymmetry, poor circumscription, irregular nests, pagetoid scatter, pleomorphism, and dermal mitoses; Breslow thickness is measured from the granular layer to the deepest tumor cell.

Clark levels: I is epidermis, II is papillary dermis, III fills papillary dermis, IV extends into reticular dermis, and V invades subcutaneous fat.

Immunohistochemistry for melanocytic lesions uses S100, SOX10, Melan-A, and HMB-45; HMB-45 expression typically diminishes with depth in benign nevi, while PRAME positivity favors melanoma.

Dysplastic nevi show lentiginous hyperplasia, lamellar fibrosis, bridging nests, and cytologic atypia; they are markers of increased melanoma risk rather than obligate precursors.

Spitz nevi contain spindle or epithelioid melanocytes, Kamino bodies, and maturation; atypical Spitz tumors need molecular testing because histology alone may be insufficient.

Lentigo maligna is melanoma in situ on sun-damaged skin with atypical melanocytes along the dermoepidermal junction; invasive lentigo maligna melanoma arises from this lesion.

Sentinel lymph node evaluation uses S100, SOX10, and Melan-A immunohistochemistry; a positive node changes melanoma staging and may guide systemic therapy.

Actinic keratosis shows atypical keratinocytes confined to the lower epidermis over sun-damaged skin; full-thickness atypia indicates squamous cell carcinoma in situ, also called Bowen disease.

Invasive squamous cell carcinoma forms nests of atypical keratinocytes with keratin pearls and desmoplasia; perineural invasion, deep invasion, and poor differentiation are high-risk features.

Basal cell carcinoma shows basaloid nests with peripheral palisading, clefting, and mucin; Ber-EP4 immunohistochemistry is positive and helps distinguish it from squamous cell carcinoma.

Seborrheic keratosis has basaloid cells, horn cysts, and pseudohorn cysts; it is benign and usually shows no significant atypia.

Keratoacanthoma is a crateriform squamous proliferation with a central keratin plug and glassy cytoplasm; it may regress spontaneously but can show invasive features.

Pilomatricoma contains basaloid cells and ghost cells with frequent calcification; it typically presents as a firm nodule in children.

Sebaceous adenoma consists of sebaceous lobules with mature sebocytes; multiple sebaceous neoplasms may indicate Muir-Torre syndrome.

Merkel cell carcinoma is a neuroendocrine carcinoma with small blue cells, dotlike CK20, and CM2B4 positivity; it is aggressive and often associated with Merkel cell polyomavirus.

Mycosis fungoides shows epidermotropic atypical T cells with cerebriform nuclei, Pautrier microabscesses, and a CD4-positive phenotype with loss of CD7.

Sezary syndrome combines erythroderma, circulating Sezary cells, and blood involvement; it is the leukemic form of cutaneous T-cell lymphoma.

CD30-positive lymphoproliferative disorders include lymphomatoid papulosis and primary cutaneous anaplastic large cell lymphoma; histology and clinical course separate them.

Primary cutaneous follicle center lymphoma expresses Bcl-6 and CD20; primary cutaneous marginal zone lymphoma often expresses Bcl-2 and may show plasmacytic differentiation.

Leukemia cutis often shows a Grenz zone with perivascular or periadnexal infiltrates; immunohistochemistry for myeloperoxidase, CD43, and lineage markers helps classify it.

Mastocytosis shows dermal mast cells positive for tryptase and CD117; CD25 expression and urticaria pigmentosa are common, and systemic disease requires staging.

Langerhans cell histiocytosis shows grooved nuclei, eosinophils, and positivity for S100, CD1a, and langerin; it can involve skin, bone, and other organs.

Rosai-Dorfman disease shows emperipolesis with S100-positive and CD1a-negative histiocytes; skin involvement may occur with or without lymphadenopathy.

Dermatofibroma shows collagen trapping, epidermal hyperplasia, factor XIIIa positivity, and CD34 negativity; it is a benign fibrous histiocytoma.

Dermatofibrosarcoma protuberans is a storiform spindle cell tumor with CD34 positivity and COL1A1-PDGFB fusion; it is locally aggressive and requires wide excision.

Kaposi sarcoma contains slit-like vascular spaces, spindle cells, and HHV8 positivity; it is associated with immunosuppression and presents as violaceous plaques or nodules.

Infantile hemangioma is GLUT1 positive, distinguishing it from vascular malformations; it typically grows after birth and then involutes.

Angiosarcoma shows atypical endothelial cells with CD31 and ERG positivity and is HHV8 negative; it is aggressive and often arises on sun-damaged scalp skin.

Schwannoma has Antoni A and B areas, Verocay bodies, and S100 positivity; neurofibroma has wavy cells with S100 and CD34 positivity.

Granular cell tumor shows granular cytoplasm with S100 and CD68 positivity; it is usually benign but can require complete excision.

Direct immunofluorescence detects immune deposits in skin and helps diagnose pemphigus, pemphigoid, dermatitis herpetiformis, and lupus; salt-split skin localizes deposits.