Lentigo maligna (LM) is a slow-growing skin cancer that develops on chronically sun-damaged skin, typically appearing on the face or neck of older adults. Although the name contains “maligna,” LM is not immediately life-threatening because the cancer cells are contained. However, untreated LM carries a serious risk: it can progress into an invasive, life-threatening form called Lentigo Maligna Melanoma (LMM). The distinction between this non-invasive stage and the invasive stage determines the ultimate risk to a person’s life.
Understanding Non-Invasive Versus Invasive Stages
Lentigo Maligna is categorized as melanoma in situ, meaning “in its original place.” This signifies that the malignant pigment-producing cells (melanocytes) are strictly confined to the epidermis, the top layer of skin. Because the cancer cells have not broken through the basement membrane separating the epidermis from the dermis, they cannot access blood or lymphatic vessels. This confinement means LM poses virtually no immediate risk of metastasis (spreading to other parts of the body).
The risk changes completely when the condition progresses to Lentigo Maligna Melanoma (LMM), the invasive form. LMM is diagnosed when cancerous cells breach the basement membrane and invade the dermis. Once in the dermis, malignant cells can enter the circulatory and lymphatic systems, allowing them to travel to distant organs and tissues. This ability to metastasize makes LMM a potentially fatal disease, similar in risk to other types of invasive melanoma.
How Lentigo Maligna Progresses
The progression from non-invasive LM to invasive LMM is typically slow, often taking many years or decades. This transformation occurs when melanocytes acquire genetic mutations allowing them to overcome the basement membrane barrier. While the average time for progression has been estimated at around 28 years, this is highly variable among individuals.
Factors increasing the likelihood of transformation include larger lesion size, prolonged time without treatment, and continuous sun exposure. The estimated annual risk of an LM lesion progressing to LMM is low, around 3.5% per year. However, this risk increases significantly with lesion size; for example, lesions larger than four centimeters have a much higher chance of containing an invasive focus.
The process of invasion is not guaranteed, meaning not all LM lesions will become LMM in a person’s lifetime. However, if progression to LMM occurs, the prognosis is determined by the depth of the invasion, known as the Breslow thickness. This measurement is used just like it is for any other invasive melanoma.
Treatment Options and Long-Term Outlook
Surgical excision is the standard method for treating Lentigo Maligna, as the main goal is to eliminate the confined cancer cells before they can progress. Wide local excision involves removing the lesion along with a margin of surrounding healthy tissue. Mohs Micrographic Surgery (MMS) is often preferred, particularly for lesions on the face or neck, because it allows for precise margin control.
Mohs Micrographic Surgery
Mohs surgery involves removing thin tissue layers and immediately examining them under a microscope until no cancer cells remain at the edges. This technique allows for precise margin control, preserves healthy tissue, and is associated with a lower recurrence rate compared to conventional excision.
Non-Surgical Alternatives
For patients unsuitable for surgery or those with lesions in difficult locations, alternative non-surgical options are available. These include radiation therapy or topical immunotherapy creams, which stimulate the immune system to attack the cancer cells.
When Lentigo Maligna is detected and completely treated while it remains in the non-invasive stage, the long-term outlook is excellent. Cure rates with surgical treatment are extremely high, often approaching 100%. This definitive treatment effectively eliminates the risk of mortality associated with progression to LMM.