Lentigo maligna (LM) is classified as a form of melanoma, the most serious type of skin cancer. LM itself is a non-invasive lesion that is highly treatable when detected early. It is identified as melanoma in situ, meaning the malignant cells are confined entirely to the top layer of the skin (the epidermis). This confinement prevents the lesion from spreading to other parts of the body, making it dangerous only if allowed to progress over time.
Understanding Lentigo Maligna
Lentigo maligna typically develops on chronically sun-damaged skin, most often on the face, neck, or forearms of older adults. It appears as a large, flat, irregularly shaped patch of discolored skin, varying in shade from tan or light brown to black. Unlike a common freckle, LM lesions grow slowly over many years, sometimes reaching several centimeters in diameter before being noticed. LM is strictly limited to the epidermis, the outermost layer of the skin. The malignant melanocyte cells proliferate horizontally within this layer but have not yet breached the basement membrane, the boundary separating the epidermis from the deeper dermis.
The Progression to Lethality
Lentigo maligna is not inherently deadly because its cancer cells are physically incapable of entering the bloodstream or lymphatic system. The threat arises exclusively if the lesion is left untreated and progresses to become invasive lentigo maligna melanoma (LMM). This transition occurs when malignant cells break through the basement membrane and penetrate the dermis. Once in the dermis, the cancer gains access to blood vessels and lymph nodes, enabling metastasis, the ability to spread to distant organs.
The rate at which LM progresses to LMM is slow, reflecting its long phase of horizontal growth. Studies suggest an estimated annual risk of progression of about 3.5% per year, which can translate to an average time of over 28 years for an untreated lesion to become invasive LMM. Progression can occur faster in some cases, highlighting the need for treatment. The size of the lesion is also a factor, as larger lesions have a higher risk of containing an invasive focus.
Once LMM is established, the prognosis shifts to a greater concern, similar to other invasive melanomas. The risk of death is directly related to the depth of invasion, or Breslow thickness, at the time of diagnosis. Early detection and treatment are essential to ensure the lesion is removed while the cancer cells are still confined to the non-lethal in situ stage.
Treatment Approaches
The goal of treating lentigo maligna is the complete removal of atypical melanocytes to prevent progression to invasive LMM. Surgical excision is the recognized standard treatment for LM, offering the highest cure rates. This involves cutting out the visible lesion along with a surrounding margin of healthy-looking skin to ensure all cancer cells are captured.
Due to LM’s common location on the face and its often indistinct borders, specialized surgical techniques are employed. Mohs micrographic surgery (MMS), or a staged excision technique, is frequently preferred for lesions in cosmetically sensitive areas. This method allows the surgeon to remove tissue layer by layer, mapping the margins and examining them immediately under a microscope. This ensures complete removal while conserving the maximum amount of healthy tissue, addressing LM’s subclinical extensions.
For patients who are not suitable surgical candidates, non-surgical alternatives are available, such as topical immunotherapy (imiquimod cream) or radiation therapy. These options can destroy the malignant cells and achieve high clearance rates for non-invasive LM. However, surgical removal with margin control is still considered the most definitive approach for long-term cure.
Long-Term Monitoring and Prevention
Following treatment, long-term monitoring is necessary due to the risk of recurrence and the higher risk of developing new melanomas elsewhere. Patients typically receive regular, full-body skin examinations performed by a dermatologist every six to twelve months. Patients are also encouraged to perform monthly self-examinations to notice subtle changes, such as a new pigmented spot or any change in the treated area.
The foundation of prevention lies in rigorous sun protection, as cumulative ultraviolet radiation exposure is the major underlying cause of LM. This involves:
- Daily application of broad-spectrum sunscreen with an SPF of 30 or higher.
- Seeking shade during peak sun hours.
- Wearing protective clothing.
- Wearing wide-brimmed hats.