Dysplastic nevus

by Jason Wasserman MD PhD FRCPC
November 20, 2024

A dysplastic nevus, also called an atypical mole, is a noncancerous type of skin growth made up of melanocytes, the cells that produce pigment and give skin its color. Dysplastic nevi look different from common moles because they have unusual shapes, sizes, or colors. While they are benign (noncancerous) and do not pose an immediate health risk, they are considered a marker for an increased risk of developing melanoma, a type of skin cancer.

What does a dysplastic nevus look like?

Dysplastic nevi often have an irregular shape with uneven or blurred borders. They may be larger than typical moles, sometimes over 5 millimeters across. Their colour is frequently uneven, with shades of brown, tan, pink, or red. They may feel flat, raised, or have a combination of textures. These differences can make dysplastic nevi stand out compared to normal moles.

What causes a dysplastic nevus?

The exact cause of dysplastic nevi is not fully understood, but they are thought to form due to a combination of genetic and environmental factors. People with a family history of dysplastic nevi or melanoma are more likely to develop them. Exposure to ultraviolet (UV) radiation from the sun or tanning beds is also a contributing factor.

How is this diagnosis made?

A dysplastic nevus is diagnosed after a skin biopsy. During this procedure, a dermatologist removes the mole, and a pathologist examines it under a microscope to identify features that indicate whether it is a common mole, a dysplastic nevus, or melanoma. The diagnosis is based on specific patterns of the cells and their behavior within the skin.

What are the microscopic features of a dysplastic nevus?

Under the microscope, a dysplastic nevus shows changes in the arrangement and appearance of melanocytes, or pigment-producing cells.

• Architectural disorder: This means the structure of the nevus does not follow the typical pattern seen in common moles. Dysplastic nevi are often larger than regular moles and may show melanocytes spreading in a lentiginous pattern as single cells or small clusters called nests. These nests can extend along the edges of the lesion, forming “shoulders” that reach beyond the main central area. The nests may bridge between elongated rete ridges, which are downward extensions of the top layer of the skin. Melanocytes may also scatter slightly above their normal location in the lower layers of the skin. Around these areas, the tissue may show fibroplasia, which is the formation of extra connective tissue and a patchy presence of immune cells called lymphocytes.
• Cytologic atypia: This refers to changes in the appearance of individual melanocytes. Their nuclei, which hold genetic material, may be larger than normal, irregularly shaped, and darker than usual (a feature called hyperchromatism). Some cells may have clumped chromatin or prominent nucleoli, structures inside the nucleus. Pathologists compare these abnormal cells to normal skin cells to determine the degree of atypia. Mitosis (cell division) is rare in dysplastic nevi, but it may raise concern for melanoma if seen.

These microscopic features allow pathologists to distinguish dysplastic nevi from normal moles and melanoma.

Grade

Pathologists grade a dysplastic nevus based on the degree of abnormality seen in the melanocytes and the overall structure of the lesion.

The grading system includes:

• Mild dysplasia: In a dysplastic nevus with mild dysplasia, the melanocytes show only slight irregularities, and the nevus looks closer to a normal mole.
• Moderate dysplasia: In a dysplastic nevus with moderate dysplasia, there are more noticeable abnormalities in the melanocytes and the architecture of the nevus.
• Severe dysplasia: In a dysplastic nevus with severe dysplasia, the melanocytes show significant atypia and architectural disorder, making it more difficult to distinguish the lesion from melanoma.

It is important to emphasize that even a severely dysplastic nevus is a benign growth, and the risk of its changing into melanoma is very low. However, having dysplastic nevi, especially multiple or severe ones, is considered an independent risk factor for developing melanoma in other areas of the skin. This is why regular skin examinations are recommended.

Margins

A margin refers to the edge of the tissue removed during a biopsy or surgery to treat a dysplastic nevus. After the procedure, the removed tissue is sent to a pathologist, who examines it under a microscope to determine whether the nevus cells extend to the edges of the sample.

If the margins are “clear” or “negative,” it means there are no nevus cells at the edges, indicating the entire lesion has likely been removed. If the margins are “positive,” it means nevus cells are present at the edges, suggesting that some of the lesion may remain in the skin. In this case, additional surgery may be recommended to remove any remaining nevus cells.

Margins are important because they help determine whether further treatment is needed. In the case of a dysplastic nevus, ensuring the lesion is entirely removed is particularly important for those with moderate or severe dysplasia, as incomplete removal may leave abnormal cells in the skin.

Can a dysplastic nevus turn into cancer?

Most dysplastic nevi will remain benign and do not turn into melanoma. While the risk of a single dysplastic nevus developing into cancer is very low, the presence of dysplastic nevi increases a person’s overall risk of developing melanoma over time. This risk is higher in individuals with multiple dysplastic nevi or a family history of melanoma. Regular skin examinations by a doctor are important to identify and treat any changes early.

Does a dysplastic nevus need to be removed?

Not all dysplastic nevi need removal, but many are excised to ensure they aren’t melanoma. The main reason for removal is to allow a pathologist to examine the entire lesion under a microscope, helping to determine if it is actually melanoma, especially in cases of severe dysplasia.

If the lesion is entirely removed and confirmed to be a dysplastic nevus, no further treatment is typically needed. However, regular skin checks are important for monitoring any new or changing lesions.

The decision to remove a dysplastic nevus depends on several factors, including its appearance, whether it is changing over time, and the patient’s overall risk of melanoma. Your doctor will discuss the best approach for your specific case.