Lentigo Maligna Melanoma

Nomenclature

Lentigo maligna melanoma (LMM) refers to a variant of melanoma that tends to occur on chronically sun-exposed skin of elderly Caucasians and is usually histopathologically characterized by a distribution of predominantly solitary units of melanocytes at the dermoepidermal junction. Both clinically (brown macules or patches) and with regard to the associated epidermal changes (hyperpigmentation, slight elongation of rete ridges), the lesions often display lentigo-like features. The noninvasive melanoma confined to the epidermis and/or hair follicles (melanoma in situ) is called lentigo maligna (LM) or LMM in situ. Some pathologists prefer to distinguish LM from LM melanoma in situ, proposing the term LM for a histopathologically more subtle (presumably “earlier”) stage in the evolution of melanoma preceding “fully developed” melanoma in situ. In this chapter, LM is used synonymously with LMM in situ, recognizing that this variant of melanoma in situ includes a spectrum of morphologic changes from low melanocyte cell density and/or relatively bland cytology to more obvious diagnostic features of melanoma in situ characterized by high cell density, disordered growth, and/or marked cytologic atypia, for which a consensus diagnosis of melanoma in situ can readily and reproducibly be achieved among a group of pathologists.

Historical terms for LM and LMM include Hutchinson melanotic freckle and circumscribed precancerous melanosis of Dubreuilh . In the late 19th century Hutchinson provided the first description of what is now known as LMM. He reported freckles on the skin of elderly that slowly enlarge and become black and eventually ulcerate. He assumed the process was infectious and called the lesions “infective senile freckles.” His work gave rise later to the terminology Hutchinson melanotic freckle . Dubreuilh reported a series of patients with similar findings and named the condition lentigo malin des viellards ( malignant lentigo of the elderly ) and la melanose circonsite precancereuse , giving rise to the term circumscribed precancerous melanosis of Dubreuilh . Other historic terms for LM include praecanceroese melanose , malignant mole , and junction nevus: nevocarcinoma .

Clinical Findings

LM typically presents as an irregular brown macule or patch on sun-damaged skin. There is a range of appearances with variations in size and color ( Fig. 12.1 ). Lesions may be dark or only light brown, amelanotic erythematous, or skin colored, clinically barely distinguishable from background skin. The majority of patients have Fitzpatrick skin types I-III. In a 10-year retrospective analysis of 7712 cases of LM from the National Cancer Data Base, 93% of patients were white non-Hispanic, 0.8% were Hispanics, and 0.5% patients were African-American. Men are slightly more affected than women (the male to female ratio is approximately 3 : 2). The head and neck region is the most commonly affected site (the cheek in women, the scalp in men), but the lesions may also occur on the upper or lower extremities or trunk. Most affected patients are elderly (median age at diagnosis is in most studies between 65 and 70 years) ( Box 12.1 ), but LM can also affect young adults with sun-damaged skin and even children in the setting of xeroderma pigmentosum.

LM usually grows slowly over years and often develops in a background of mottled hyperpigmentation. At the time of biopsy, many lesions have reached a size larger than 1 cm. Individual lesions usually manifest as asymmetric brown macules or patches with irregular borders. The clinical differential diagnosis is usually between miscellaneous pigmented epithelial lesions, including solar lentigo, lichenoid keratosis, pigmented seborrheic or pigmented actinic keratosis (AK), or pigmented squamous cell carcinoma in situ. Sometimes a melanocytic nevus is being considered clinically. Some lesions of LM are clinically amelanotic or paucimelanotic presenting, for example, as an erythematous patch, which may be biopsied to exclude basal cell carcinoma or an inflamed AK.

LMM (i.e., the association of invasive melanoma with an LM) may be clinically indistinguishable from LM, especially if tumor is thin (e.g., depth is less than 0.5 mm), and histopathologically manifest only as microinvasion confined to the papillary dermis. Thicker lesions may be clinically noticeable as papules, nodules, or areas of induration or ulcer or be associated with abnormal vascular patterns. Although LM tends to grow slowly and may on occasion not become invasive over the course of a decade or more, some lesions may progress quickly to invasive melanoma and may metastasize and become lethal. Once invasion is identified, the same prognostic parameters apply as for other melanomas.

Dermoscopy has emerged as a useful adjunct to improve the accuracy of clinically diagnosing LM or LMM from clinical mimickers. Dermoscopy features favoring LM include asymmetrically pigmented or gray-colored follicular openings, perifollicular dots and granules giving rise to an annular pattern, and angulated pigmented lines that join to create zig-zag lines or polygonal structures such as rhomboids.

Histopathologic Findings