Lentigo maligna melanoma (LMM) is a type of invasive skin cancer that develops from a slow-growing precursor called lentigo maligna, a flat, irregularly pigmented patch that forms on sun-damaged skin. It accounts for a meaningful share of melanoma diagnoses and tends to appear on areas with years of cumulative sun exposure, particularly the face, ears, and neck. What makes LMM distinct from other melanomas is its unusually long pre-invasive phase, which can last years or even decades before the abnormal cells push deeper into the skin.
How Lentigo Maligna Becomes Melanoma
Lentigo maligna starts as an “in situ” melanoma, meaning the abnormal pigment-producing cells (melanocytes) are confined to the outermost layer of skin. At this stage, the lesion has no ability to spread to other parts of the body. Over time, if untreated, those abnormal cells can begin growing downward into the deeper layer of skin called the dermis. Once that happens, the lesion is reclassified as lentigo maligna melanoma, and it carries the same potential for spread as any other invasive melanoma.
The invasion tends to be superficial at first, with small clusters of spindle-shaped cells appearing just below the surface. As the invasive component grows, the lesion may develop a raised or nodular area within what was previously a flat patch. That change in texture is one of the key warning signs that a lentigo maligna has progressed.
Who Gets It and Why
LMM is strongly linked to cumulative, long-term sun exposure rather than the intermittent sunburn pattern associated with other melanoma types. This is why it overwhelmingly appears on chronically sun-exposed areas like the face, and why it’s diagnosed most often in older adults. Fair-skinned individuals with decades of UV damage are at highest risk.
Compared to superficial spreading melanoma, the most common melanoma subtype, LMM behaves differently from the start. Superficial spreading melanoma typically shows up on the trunk and lower limbs in somewhat younger patients and grows more quickly. LMM favors the face in older patients and has a much longer in situ phase. Research comparing the two types at the cellular level has confirmed that superficial spreading melanoma cells proliferate faster and generate more blood vessel growth, which helps explain why lentigo maligna can sit on the skin for years before becoming dangerous.
What It Looks Like
In its early, in situ phase, lentigo maligna appears as a flat, irregularly shaped brown or tan patch. The color is often uneven, with areas of light brown, dark brown, and sometimes black or slate-gray. The borders tend to be blurred and irregular. Because it develops on sun-damaged skin, it can easily be mistaken for a harmless age spot or sun spot.
A few features help distinguish it. When examined under a dermatoscope (a specialized magnifying tool), lentigo maligna and LMM show characteristic patterns: asymmetric pigment around hair follicles, dark rhomboidal (diamond-shaped) structures, and slate-gray dots or globules. Studies have found that the combination of asymmetric follicular pigmentation, dark rhomboidal structures, and slate-gray dots has high accuracy for identifying these lesions. In one study, asymmetric pigmented follicular openings were present in about 68 to 82% of confirmed cases, and dark dots appeared in nearly 87%.
The shift from in situ to invasive is often signaled by a new raised area, a darker nodule, or a change in the overall texture of the patch. Any thickening within a previously flat pigmented lesion on sun-damaged skin warrants prompt evaluation.
Diagnosis and Biopsy
Confirming LMM requires a skin biopsy. Several techniques are used depending on the size and location of the lesion: a shave biopsy (removing a thin layer), a punch or incisional biopsy targeting the thickest or most concerning area, an excisional biopsy with narrow margins, or a broad saucerization that removes the entire visible lesion with enough depth to evaluate properly. Because these lesions often sit on the face, where removing large amounts of tissue has cosmetic consequences, the biopsy approach is tailored to each case.
Under the microscope, pathologists look for abnormal melanocytes spreading along the base of the outer skin layer, a hallmark of the in situ phase. When invasion has occurred, they look for clusters of spindle-shaped cells that have pushed into the upper dermis. The depth of that invasion, measured in millimeters, is the single most important factor in determining prognosis and guiding treatment.
Treatment Options
Surgery
Surgical removal is the standard treatment for both lentigo maligna and LMM. The challenge is that these lesions often have invisible extensions beyond what’s visible to the naked eye, particularly on facial skin. Two main surgical approaches are used: wide local excision, where a margin of healthy-looking skin is removed around the visible lesion, and Mohs micrographic surgery, where tissue is removed in thin layers and examined under a microscope in real time until no abnormal cells remain at the edges.
Mohs surgery is particularly appealing for facial lesions because it conserves as much healthy tissue as possible while thoroughly checking the margins. A large study analyzing over 22,800 cases treated between 2000 and 2019 found no significant difference in disease-specific survival between wide local excision and Mohs surgery, suggesting both are effective. Mohs surgery may offer an advantage in tissue preservation on cosmetically sensitive areas like the nose, eyelids, and ears.
Topical Immunotherapy
For lentigo maligna that hasn’t yet become invasive, a prescription cream called imiquimod is sometimes used as an alternative or supplement to surgery. It works by stimulating the skin’s immune system to attack abnormal cells. This is an off-label use, but it has been studied extensively.
A systematic review of long-term studies found clinical clearance rates between 64% and 97%, with the variation depending on the study and patient population. Recurrence rates ranged from 0% to about 21%. In the largest retrospective series, clearance reached 97% with a recurrence-free survival rate of 82% at five years. Some smaller studies reported zero recurrences over follow-up periods averaging five or more years. Imiquimod is typically applied five times per week for about 12 weeks. It’s generally considered when surgery would be difficult or when patients are not good surgical candidates, and it’s sometimes used after surgery to treat any remaining abnormal cells at the margins.
Radiation Therapy
Radiation is an option for patients who can’t undergo surgery due to age, other health conditions, medications like blood thinners, or when surgery would cause significant functional or cosmetic problems. European guidelines list radiation as a treatment option for inoperable LMM.
Local recurrence rates with radiation therapy range from 0% to about 31%, with most studies reporting rates comparable to surgical series. Studies focused specifically on LMM lesions (rather than the in situ form) have reported recurrence rates between 0% and 17%. The wide range reflects differences in radiation technique, dose, and patient selection across studies.
Prognosis
The outlook for LMM depends heavily on how deep the melanoma has grown at the time of diagnosis. When caught in the in situ phase, before any invasion has occurred, the prognosis is excellent because the cancer has no access to blood vessels or lymph nodes and cannot spread. Once invasion occurs, LMM carries the same stage-for-stage prognosis as other melanoma subtypes. A thin LMM caught early has a very favorable outcome, while a thicker one carries the same risks as any other invasive melanoma of equivalent depth.
The silver lining of LMM is its long in situ phase. Because lentigo maligna can exist on the skin for years before becoming invasive, there’s a wide window for detection and treatment. The key is recognizing that a changing, irregularly pigmented patch on sun-damaged skin, particularly on the face or scalp of an older adult, isn’t just a cosmetic concern. Early biopsy of suspicious lesions catches most cases before invasion occurs, when treatment is straightforward and cure rates are high.