Keratoacanthoma

by Jason Wasserman MD PhD FRCPC and Zuzanna Gorski MD
December 19, 2023

Keratoacanthoma is a rapidly growing but benign (noncancerous) type of skin tumour. It typically involves hair-bearing skin on the face although other parts of the body can occasionally be involved.

What causes keratoacanthoma?

Keratoacanthoma is believed to be caused by excessive sun exposure. However, in many cases, the tumour only starts to grow after the skin is involved in a secondary event such as trauma, insect bite, medical procedure, or chemical exposure.

What are the symptoms of keratoacanthoma?

The symptoms of keratoacanthoma are unique and aid in the diagnosis. These include the sudden appearance of a smooth, dome-shaped nodule that grows rapidly over several months. Growth typically stops and the tumour remains stable for months before spontaneously decreasing in size. In many cases, the tumour can disappear completely without any treatment.

Is keratoacanthoma a type of skin cancer?

Keratoacanthoma is not a type of skin cancer. Despite its rapid growth, it is believed to be a reactive (noncancerous) process and the cells in the tumour are not capable of metastasizing (spreading) to lymph nodes or other parts of the body. However, some tumours can grow to be very large and their growth can damage surrounding normal tissues.

What does keratoacanthoma look like under the microscope?

Under microscopic examination, keratoacanthoma is made up of well differentiated squamous cells that closely resemble the squamous cells normally found in the skin. The cells do not show significant cytologic atypia (which means they are normal-looking) and mitotic figures (cells dividing to create new cells) are usually only seen at the very periphery of the tumour.

The tumour shows both exophytic and endophytic growth which means it grows both up from the surface of the skin and down into the underlying tissue. The downward growth is often described as bowl-shaped as it is very smooth and round. One characteristic feature of this tumour is that its centre is filled with keratotic debris (dead mature squamous cells).

Inflammatory cells, predominantly lymphocytes, are typically seen surrounding the tumour in a pattern pathologists describe as lichenoid. More aggressive features such as perineural invasion, lymphovascular invasion, and necrosis are rarely seen in keratoacanthoma. The absence of these features helps distinguish this tumour from well differentiated squamous cell carcinoma, a type of skin cancer, which often shows these features.

What is the difference between keratoacanthoma and squamous cell carcinoma?

Despite its rapid growth, keratoacanthoma is considered a benign (noncancerous) tumour that can spontaneously disappear and is cured by surgery alone. In contrast, squamous cell carcinoma is a malignant (cancerous) tumour that can metastasize (spread) to lymph nodes and other parts of the body.

About this article

This article was written by doctors to help you read and understand your pathology report for keratoacanthoma.  Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

Other helpful resources

Atlas of Pathology