seborrheic keratosis

**Definition/Epidemiology:** Benign proliferation of epidermal keratinocytes (basaloid cells). Most common benign skin tumor in older adults (typically >40 yrs); incidence increases with age. Often familial (autosomal dominant tendency). Lesions are found on the trunk, face, and extremities; they spare the palms and soles.

**Pathophysiology:** Clonal expansion of epidermal keratinocytes. Activating mutations in FGFR3 (and PIK3CA) are found in many lesions. Sun exposure may contribute, but the lesions are also common in non-sun-exposed sites. HPV is not a cause.

**Clinical features:** - Well-demarcated, round-to-oval, waxy, greasy, "stuck-on" papules or plaques. - Color ranges from tan to brown to black. - Surface is verrucous or velvety with a cerebriform appearance. - Usually asymptomatic. Some are pruritic, or irritated by trauma or clothing. - Dermoscopy shows comedo-like openings and milia-like cysts. - Dermatosis papulosa nigra is a variant seen in darker-skinned individuals, with multiple small lesions on the face.

**Histology:** - Hyperkeratosis, acanthosis, and papillomatosis. - Horn cysts (keratin-filled invaginations) and pseudo-horn cysts. - Proliferation of basaloid cells, with a sharply demarcated, "stuck-on" architecture. - Melanin variably increased. Melanocyte number is normal.

**Key association – Sign of Leser-Trélat:** Sudden onset or rapid increase in the number and size of seborrheic keratoses. It is a paraneoplastic sign, usually of GI adenocarcinoma (stomach, colon), and also lymphoma. It is related to tumor secretion of growth factors (TGF-α, which acts on EGFR). Compare to acanthosis nigricans, which is also a paraneoplastic marker associated with GI adenocarcinoma.

**Differential diagnosis:** Melanoma (the key mimic), pigmented basal cell carcinoma, actinic keratosis, solar lentigo, verruca vulgaris, and nevus. Consider biopsy when the lesion is atypical, changing, bleeding, or ulcerated.

**Management:** Benign; no treatment is needed. Treat for cosmetic or symptomatic reasons or when the diagnosis is uncertain: cryotherapy (liquid nitrogen), curettage, shave excision, or electrodesiccation. Send the specimen for histology if there is any doubt.

**High-yield board pearls:** - "Stuck-on," greasy, waxy plaque in an elderly patient → seborrheic keratosis. - Sudden eruption of many lesions → Leser-Trélat → look for GI adenocarcinoma. - Not premalignant (unlike actinic keratosis, which can progress to SCC). - Histology: horn cysts and acanthosis with basaloid cells.