Lentigo Maligna Melanoma: Slow-Growing Melanoma on Sun-Damaged Skin

Lentigo maligna melanoma (LMM) is one of the four major subtypes of cutaneous melanoma, the most dangerous form of skin cancer. It develops from lentigo maligna, which is a form of melanoma in situ, meaning the abnormal melanocytes are confined to the outermost layer of the skin (the epidermis) and have not yet invaded deeper tissues. When lentigo maligna progresses and the malignant cells break through the basement membrane into the dermis, it becomes lentigo maligna melanoma. This subtype accounts for approximately 4-15 percent of all melanomas and is distinguished by its association with chronic cumulative sun damage rather than intermittent intense exposure.

Lentigo maligna melanoma predominantly affects older adults, with the average age at diagnosis being around 65 to 70 years. It is most common in individuals with fair skin, light eyes, and a lifetime history of significant sun exposure, particularly those who have spent many years working or recreating outdoors. The condition is more frequently diagnosed in men than women, possibly due to historical differences in occupational sun exposure. People with a history of other sun-related skin damage such as actinic keratoses, solar lentigines, and non-melanoma skin cancers are at higher risk.

Lentigo maligna typically begins as a flat, irregularly shaped tan or brown patch on chronically sun-exposed skin, most commonly on the face, ears, neck, or scalp. Over time, the lesion gradually enlarges and may develop increasingly irregular borders and variations in color, including shades of dark brown, black, pink, or even areas of depigmentation where the immune system has attacked the tumor. The surface remains flat during the in situ phase, which can last for years or even decades. If the lesion develops a raised nodular component, this may indicate progression to invasive lentigo maligna melanoma.!!

The critical concern with lentigo maligna is its potential to progress from an in situ lesion to an invasive melanoma. While the in situ phase can last for many years, with some lesions remaining non-invasive for a decade or longer, there is no reliable way to predict which ones will progress. Studies suggest that approximately 5 to 50 percent of lentigo maligna cases will eventually develop an invasive component if left untreated, though the wide range reflects uncertainty in the literature. Once invasion occurs, the melanoma gains access to blood vessels and lymphatic channels, creating the potential for metastatic spread.

Diagnosing lentigo maligna and lentigo maligna melanoma requires a combination of clinical examination and histopathological analysis. Dermoscopy, a technique that uses a special magnifying instrument to examine skin structures not visible to the naked eye, can reveal characteristic patterns such as asymmetric pigmented follicular openings, annular-granular structures, and rhomboidal structures. A biopsy is essential for definitive diagnosis, and an excisional or broad shave biopsy is preferred to ensure adequate tissue sampling. The pathologist evaluates the biopsy to determine whether the melanocytes are confined to the epidermis or have invaded into the dermis.

Surgical excision is the gold standard treatment for both lentigo maligna and lentigo maligna melanoma. For in situ disease, wide local excision with 5 to 10 millimeter margins is typically recommended, though achieving clear margins can be challenging due to the poorly defined borders common with these lesions. Mohs micrographic surgery or staged excision techniques are increasingly used, particularly for lesions on the face where tissue conservation is important. For invasive lentigo maligna melanoma, excision margins are determined by the Breslow thickness of the tumor, and sentinel lymph node biopsy may be recommended for deeper tumors.

The prognosis for lentigo maligna melanoma depends heavily on the stage at which it is diagnosed. When caught in the in situ phase, the cure rate with adequate surgical excision is excellent, approaching nearly 100 percent.!! For invasive lentigo maligna melanoma, the prognosis is generally comparable to other melanoma subtypes of similar thickness and stage, with thinner tumors carrying a significantly better prognosis.

The five-year survival rate for thin lentigo maligna melanomas is very high. However, local recurrence rates can be higher than for other melanoma subtypes due to the difficulty in achieving clear surgical margins on chronically sun-damaged skin.

Given the slow-growing nature of lentigo maligna, regular monitoring offers an excellent opportunity for early detection before invasion occurs. Anyone with a history of significant sun exposure, particularly older adults with fair skin, should perform regular self-examinations paying close attention to any flat, irregularly pigmented patches on sun-exposed skin. Professional skin examinations with dermoscopy are especially valuable for detecting subtle changes that may not be apparent to the untrained eye. Skinscanner can be a valuable tool for tracking pigmented lesions over time, helping you document changes in size, shape, or color that should prompt a visit to your dermatologist.