Benign tumors and cysts of the epidermis

A clinical image atlas for entities throughout the text can be found in the on-line content for this book.

Benign acanthomas

Acanthomas are benign cutaneous neoplasms characterized by an expansion of the epidermis. The acanthoma may be composed of clones of cells that displace or compress the preexisting epidermis. In contrast to the reactive acanthosis seen in inflammatory disorders, the rete ridge pattern is commonly ablated by the neoplastic tissue of an acanthoma.

Seborrheic keratoses

Seborrheic keratoses are acanthomas composed of small polygonal keratinocytes about the size of acrosyringeal keratinocytes (the cells that make up the intraepidermal portion of the eccrine duct). The cells are typically smaller than the cells of the surrounding epidermis, and they are commonly pigmented. Architectural subtypes of seborrheic keratoses include acanthotic, hyperkeratotic, reticulated, and clonal. Any of these subtypes may be pigmented, irritated (spindling of cells and squamous eddy formation), or inflamed (usually lymphoid inflammation). Melanoacanthoma is a distinct subtype of seborrheic keratosis composed of small keratinocytes and dendritic melanocytes.

Seborrheic keratoses produce a characteristic loose lamellar “shredded-wheat” stratum corneum. Exceptions include irritated or inflamed seborrheic keratosis. Instead of the characteristic loose lamellar horn, irritated or inflamed seborrheic keratoses produce a compact brightly eosinophilic parakeratotic stratum corneum. Adjacent unaffected areas of the seborrheic keratosis still produce the characteristic loose lamellar stratum corneum, and it is common to see remnants of loose stratum corneum above areas of compact stratum corneum. Melanoacanthomas produce a deeply eosinophilic compact parakeratotic stratum corneum, even when they are not irritated or inflamed.

Seborrheic keratoses may express BCL-2, a marker associated with resistance to programmed cell death (apoptosis). Activating point mutations in the gene encoding fibroblast growth factor receptor 3, a tyrosine kinase receptor, are also common in seborrheic keratoses.

Acanthotic seborrheic keratosis

Key features

• Broad sheets of small polygonal keratinocytes with intervening horn cysts

• Loose lamellar “shredded-wheat” or “onion-skin” keratin

• Commonly pigmented

Fig 2-1 Acanthotic seborrheic keratosis

Acanthotic seborrheic keratoses are composed of broad sheets of cells with intervening horn cysts or pseudohorn cysts. Horn cysts are completely encased within the acanthoma, whereas pseudohorn cysts open to the surface. Like other seborrheic keratoses, they may become irritated or inflamed.

Fig 2-2 Pigmented acanthotic seborrheic keratosis

Fig 2-3 Irritated acanthotic seborrheic keratosis

Fig 2-4 Inflamed acanthotic seborrheic keratosis

Fig 2-5 Hyperkeratotic seborrheic keratosis

Hyperkeratotic seborrheic keratosis

Key features

• Tall stacks of loose lamellar “shredded-wheat” keratin

• Papillomatosis – hills and dales that may produce a “church-spire” appearance

Fig 2-6 Inflamed hyperkeratotic seborrheic keratosis

The tall stacked stratum corneum is typically much thicker than the epidermis. As in other forms of seborrheic keratosis, the keratin has a loose lamellar “shredded-wheat” appearance unless the lesion has become irritated or inflamed. Papillomatosis is characteristic, but variable in degree. Horn cysts are inconspicuous or absent.

Clonal seborrheic keratosis is characterized by islands of small keratinocytes with uniform bland nuclei. The nests are embedded within the epidermis. Sometimes, the nests are large enough that the normal epidermis is reduced to thin strands separating the large nests. Horn cysts are usually absent. The nests may demonstrate pigment. There may be squamous eddies (irritated seborrheic keratosis), lymphocytes (inflamed seborrheic keratosis), or both. In contrast to Bowen’s disease, the cells are uniform and atypia is absent. In contrast to hidroacanthoma simplex, no ducts are present within the clones.

Differential Diagnosis

Hidroacanthoma simplex

Key features