Dysplastic nevus

by Allison Osmond MD FRCPC and Archan Kakadekar MD
March 13, 2023

What is a dysplastic nevus?

A dysplastic nevus is a non-cancerous type of skin tumour made up of specialized cells called melanocytes. Another name for this type of growth is an atypical mole.

Is a dysplastic nevus a type of cancer?

No. A dysplastic nevus is not a type of skin cancer. However, some dysplastic nevi can turn into a type of skin cancer called melanoma over time. The risk of developing cancer is higher for dysplastic nevi that show severe dysplasia and for nevi that are greater than 5 mm in size. People with more than one dysplastic nevus also have a higher risk of developing melanoma over time.

Is a dysplastic nevus a precancerous tumour?

A dysplastic nevus is not considered to be a precancerous tumour because most will not turn into cancer over time. However, dysplastic nevi with severe dysplasia should be treated as precancerous as they are associated with a higher risk of developing a type of skin cancer called melanoma over time.

What causes a dysplastic nevus?

Like other types of melanocytic nevi, dysplastic nevi are associated with prolonged sun exposure, especially in people with light skin.

What does a dysplastic nevus look like when examined without a microscope?

Most dysplastic nevi are flat, and they can show a variety of colours including black, brown, red, and blue. The border between the tumour and the surrounding skin is often irregular (there is no clear border) which can make it hard to tell where the nevus ends, and the normal skin begins. When examined without a microscope, a dysplastic nevus can look similar to a melanoma. For that reason, your doctor may suggest removing the tumour so that it can be examined by a pathologist under a microscope.

How is this diagnosis made?

This diagnosis can only be made after a tissue sample is removed and examined under the microscope by a pathologist. This usually involves removing the entire nevus in one piece along with a small amount of surrounding normal-appearing skin. In order to make this diagnosis, pathologists look for both architectural features and cytologic features. Architectural features describe how the abnormal melanocytes in the nevus connect together and how the tissue around the nevus is changing or responding to the growth. Cytologic features describe how the individual melanocytes look including their shape, size and colour.

Architectural features assess when examining a dysplastic nevus

• Irregular nests – The melanocytes in an ordinary (non-dysplastic) nevus are usually found in small round groups called nests. In a dysplastic nevus, the nests often have an irregular shape (they are less round) and they may be larger than the nests seen in an ordinary nevus.
• Bridging or fusing of nests – In an ordinary (non-dysplastic nevus), the nests of melanocytes do not touch. However, the nests in a dysplastic nevus may grow in a way where they start to touch or even join. Pathologists describe this as bridging or fusing of the nests.
• Lentiginous growth – Lentiginous is a word pathologists use to describe melanocytes spreading side by side as single cells along the bottom of the epidermis. A lentiginous pattern of growth is not normally seen in an ordinary (non-dysplastic) nevus.
• Shouldering – The melanocytes in the epidermis of an ordinary (non-dysplastic) nevus do not generally extend beyond the melanocytes deeper in the dermis. However, in a dysplastic nevus, the melanocytes may grow beyond the cells in the dermis creating the appearance of a shoulder at the top of the nevus.
• Dermal lamellar fibroplasia – This change happens in the dermis immediately below the nevus. In a dysplastic nevus, the stroma becomes darker and wavier than normal. Pathologists describe this as lamellar fibroplasia and it is an abnormal finding.

Cytologic features assessed when examing a dysplastic nevus

• Size of the nucleus – The nucleus is the part of the cell that holds the cells’ genetic material (DNA). The melanocytes in a dysplastic nevus may have nuclei that are larger than normal melanocytes.
• Nucleoli – Nucleoli are small round balls of genetic material inside the nucleus. The melanocytes in a dysplastic nevus may have easy-to-see nucleoli. Pathologists describe these nucleoli as prominent because they can be seen even at low magnification.
• Mitotic figures – Cells divide to create new cells. A cell that is in the process of dividing to create a new cell is called a mitotic figure. Mitotic figures are not usually seen in non-dysplastic nevi. In contrast, dysplastic nevi may have one or more mitotic figures.
• Atypia – Atypia is word pathologists use to describe a cell that looks different from the normal, healthy cells in that location. Atypia in a nevus is a sign of abnormal growth and is more often seen in a dysplastic nevus. Atypia in a nevus can mean that the melanocytes were larger, darker, or had an irregularly shaped nucleus compared to normal melanocytes.

What is the difference between a junctional dysplastic nevus and a compound dysplastic nevus?

Pathologists divide dysplastic nevi into two types – junctional and compound – based on where the melanocytes are found in the skin. In a junctional dysplastic nevus, abnormal melanocytes are only found in a layer of tissue at the surface of the skin called the epidermis. In a compound dysplastic nevus, abnormal melanocytes are found both in the epidermis and in a deeper layer of tissue called the dermis.

What does it mean if a dysplastic nevus is described as showing mild, moderate, or severe dysplasia?

Pathologists separate dysplastic nevi into three levels – mild, moderate, and severe – based on the types of architectural and cytologic features seen when the tumour is examined under the microscope. Nevi with mild and moderate dysplasia are also called low grade while nevi with severe dysplasia are called high grade. The level of dysplasia is important because the risk of developing melanoma is much greater for dysplastic nevi with severe (high grade) dysplasia.