Brown Papules and a Plaque on the Calf

The Diagnosis: Irritated Seborrheic Keratosis

Biopsies of one of the protruding papules and the underlying plaque were performed. The specimen from the papule showed hyperkeratosis, acanthosis, papillomatosis, and a flattened dermoepidermal junction with demarcated horizontal margin, which demonstrated apparent upward growth of the epidermis. Moderate lymphocytic infiltration in the upper dermis also was observed (Figure, A). The histologic findings of the plaque showed acanthosis, several pseudohorn cysts, hyperpigmentation of the basal layer, and a horizontal demarcation of the dermoepidermal junction (Figure, B).

A biopsy of a papule showed hyperkeratosis, acanthosis, papillomatosis, and a flattened dermoepidermal junction with demarcated horizontal margin. Moderate lymphocytic infiltration in the upper dermis was noted (A)(H&E, original magnification ×100). A biopsy of the plaque showed acanthosis, several pseudohorn cysts, hyperpigmentation of the basal layer, and a horizontal demarcation of the dermoepidermal junction (B)(H&E, original magnification ×100).

Seborrheic keratosis is the most common benign epidermal tumor of the skin with variable appearance.¹ It usually begins with well-circumscribed, dull, flat, tan or brown patches that then grow into waxy verrucous papules.¹ There are many clinicopathologic variants of SK such as common SK, stucco keratosis, and dermatosis papulosa nigra in clinical variation, as well as acanthotic, hyperkeratotic, clonal, reticulated, irritated, and melanoacanthoma subtypes based on histological variation.^2,3

Seborrheic keratosis is a tumor of keratinocytic origin. Although genetics, sun exposure,⁴ and human papillomavirus infection⁵ are thought to be causative factors, the precise etiology of SK is unknown.¹

The histology of SK shows monotonous basaloid tumor cells without atypia. It generally is comprised of focal acanthosis and papillomatosis with a sharp flat base. Intraepithelial horn pseudocysts are notable features of SK and increased melanin often is seen.^2,6

Irritated SK is a histologic variant of SK that has been mechanically or chemically irritated or is involved in immunologic responses. Histologically, the dermis underlying an SK lesion filled with a dense lymphocytic infiltration is characteristic.^1,2

For symptomatic or cosmetically undesirable lesions, complete removal of the lesion is the preferred treatment. Cryotherapy, electrodesiccation followed by curettage, curettage followed by desiccation, laser ablation, and surgical excision are effective treatments.¹