Clonal highlights: Clonal seborrheic keratoses often demonstrates p16 expression.

Area of Science:

• Dermatopathology
• Oncology
• Immunohistochemistry

Background:

• Distinguishing Bowen disease (BD) from benign seborrheic keratosis (SK), particularly clonal seborrheic keratosis (CSK), presents histological challenges.
• p16 is typically positive in BD and negative in SK, but its expression in CSK is less understood.

Purpose of the Study:

• To investigate p16 immunohistochemistry expression patterns in CSK, SK, and BD.
• To clarify the diagnostic significance of p16 staining in differentiating these skin lesions.

Main Methods:

• p16 immunohistochemistry was performed on 14 CSK, 12 SK, and 18 BD samples.
• The pattern and degree of p16 expression, along with inflammation, were analyzed for each lesion type.

Main Results:

• p16 staining in CSK showed diffuse, patchy/diffuse, or clustered single-cell positivity in 100% of cases.
• Bowen disease (BD) exhibited diffuse positivity in 67% of cases.
• Seborrheic keratosis (SK) showed focal positivity in 50% of cases, with the remainder negative or showing scattered single cells.

Conclusions:

• p16 positivity confined to clonal nests in CSK is considered a normal finding.
• Isolated p16 positivity within clonal nests of CSK, without other atypical histological features, should not be interpreted as indicative of BD.