A 65-year-old woman presents with a slowly enlarging, pink, papule on her lower leg. Shave biopsy reveals a sharply circumscribed epidermal lesion with psoriasiform hyperplasia, pallor of keratinocytes, and neutrophils within the epidermis. PAS stain is negative for fungal organisms. Histology is shown below.
Which of the following is the most likely diagnosis?
A. Clear cell acanthoma
B. Psoriasis vulgaris
C. Bowen’s disease
D. Dermatophytosis
Rationale:
Clear cell acanthoma (CCA) is a benign epidermal tumor typically presenting as a slowly enlarging, pink to red, dome-shaped papule or plaque, often on the lower extremities. Histologically, it is characterized by sharp demarcation from adjacent skin, psoriasiform hyperplasia, clear keratinocytes due to intracellular glycogen, and frequent neutrophilic microabscesses in the epidermis. It often mimics psoriasis or infection, both clinically and histologically.
Correct answer: A. Clear cell acanthoma
The key histologic clues are clear cytoplasm of keratinocytes, sharply demarcated psoriasiform architecture, and intralesional neutrophils, with no evidence of atypia or fungal organisms. The pallor is due to intracellular glycogen, which is PAS-positive and diastase-sensitive. Clinically and histologically, CCA can resemble psoriasis, but the solitary nature and absence of other psoriatic features point toward CCA.
Incorrect answer choices:
B. Psoriasis vulgaris often shows confluent parakeratosis, thinned suprapapillary plates, and regular elongation of rete ridges with neutrophils in the stratum corneum (Munro microabscesses), but it lacks the clear keratinocytes with glycogen and is usually not sharply circumscribed or solitary.
C. Bowen’s disease (squamous cell carcinoma in situ) exhibits full-thickness epidermal atypia, disordered maturation, and atypical mitotic figures, all of which are absent in this biopsy.
D. Dermatophytosis can show neutrophils and pallor, but fungal elements would be visible on PAS or GMS stains. Also, the keratinocyte pallor is due to cytoplasmic edema, not glycogen.
Additional reading at Barnhill's Dermatopathology Chapter 26: Tumors of the Epidermis