Lentigo maligna

Jason Wasserman MD PhD FRCPC
December 9, 2024

Lentigo maligna is a type of melanoma in situ, which means it is a very early stage of skin cancer that has not yet spread beyond the top layer of the skin (the epidermis). It typically develops in areas of the skin that have been exposed to the sun for many years, such as the face, neck, and arms. This type of cancer grows very slowly and can remain in this early stage for years before progressing to an invasive melanoma called lentigo maligna melanoma.

What are the symptoms of lentigo maligna?

Lentigo maligna often appears as a flat or slightly raised patch of discoloured skin.

Key features include:

• A brown, black, or tan patch that may have areas of lighter or darker pigmentation.
• An irregular shape with uneven or blurred borders.
• A patch that slowly grows over time.
• A patch that may feel rough or scaly but usually does not cause pain, itching, or bleeding.

Because lentigo maligna often resembles other benign skin spots, such as age spots or freckles, it’s important to monitor changes in size, colour, or shape and consult a doctor if these changes occur.

What causes lentigo maligna?

Lentigo maligna is primarily caused by chronic sun exposure, which damages the DNA of the melanocytes, the pigment-producing cells in the skin. People with lighter skin tones, a history of significant sun exposure, or frequent sunburns are at higher risk of developing this type of cancer. The risk increases with age and is most common in individuals over 50. Other risk factors include a weakened immune system or a personal history of skin cancer.

What is the difference between lentigo maligna and lentigo maligna melanoma?

Lentigo maligna is an early-stage melanoma that is confined to the top layer of the skin (the epidermis). It has not yet invaded deeper layers of the skin or spread to other parts of the body. In contrast, lentigo maligna melanoma develops when the cancer cells grow downward into the dermis, the thicker middle layer of the skin. This progression marks the transition to invasive melanoma, which has the potential to metastasize (spread) to other areas of the body.

How long does it take for lentigo maligna to turn into melanoma?

Lentigo maligna grows very slowly and can remain in the in situ stage for months or even years before progressing to lentigo maligna melanoma. However, the exact time it takes to transition varies widely between individuals. Regular monitoring and early treatment of this type of cancer significantly reduce the risk of progression to invasive melanoma.

How is this diagnosis made?

The diagnosis of lentigo maligna is typically made after a skin biopsy, where a small sample of the suspicious lesion is removed and examined under a microscope by a pathologist. The biopsy helps confirm the presence of abnormal melanocytes in the epidermis and rules out other conditions. Additional tests, such as dermoscopy (a technique to examine the skin under magnification), may help identify characteristic features before the biopsy.

Microscopic features

The microscopic features of lentigo maligna include:

• Abnormal melanocytes: These pigment-producing cells appear larger, darker, and more irregular than normal melanocytes.
• Epidermal involvement: The abnormal melanocytes are confined to the epidermis and spread along the basal layer of the skin.
• Pagetoid spread: Melanocytes may extend upward into the upper layers of the epidermis.
• Solar elastosis: The underlying dermis (lower skin layer) often shows sun damage, with a buildup of abnormal elastic fibers caused by chronic sun exposure.
• Sparse inflammation: Mild inflammation may be present around the lesion, but it is usually not prominent.

What is the prognosis for a person diagnosed with lentigo maligna?

The prognosis of lentigo maligna is excellent when diagnosed and treated early. Since it is confined to the top layer of the skin, it can often be removed entirely with surgery or other treatments. Once treated, the risk of recurrence is low, although lifelong monitoring is recommended to check for new lesions or recurrences.

If lentigo maligna progresses to lentigo maligna melanoma, the prognosis depends on the depth of invasion and whether it has spread to other body parts. Early detection and regular skin checks are essential for maintaining a good outcome.