Lentigo Maligna Melanoma (LMM)
Lentigo maligna melanoma (LMM) is the least common type (about 4%) and the slowest growing. It generally has a long radial spread time often reaching large diameters of 3-6 cm or greater and confined to the upper skin layer (epidermis). This is the first stage called lentigo maligna and can last for years. During this stage the melanoma remains in situ. For this reason metastasis is rare.
The second stage is called lentigo maligna melanoma and begins when the existing lentigo maligna enters a vertical growth phase.
It is most often found in Caucasians in their 70's, especially women over the age of fifty. Characteristics include chronically sun-exposed locations on the head and neck with the nose and cheek being primary sites. LMM was 74 times more common on the face than on skin outside of the head-neck area (Desmond et al, 2003). Lesions often resemble a very large "liver spot" and for this reason are often ignored. Typical presentation is a tan-colored spot with variations of brown, black and gray. Flat to touch. Asymmetric large shape with irregular, convoluted borders.
While prognosis for LM is generally good once it enters the second stage and becomes LMM the outcome decreases. There is also a rare type of melanoma called desmoplastic that grows in association with LMM that has a poor prognosis. Approximately 50% of desmoplastic melanomas are amelanotic and may appear scar-like when next to a LMM. Lesions that have both pigmented and non-pigmented segments should be examined without delay.