An easily missed melanoma

Lentigo maligna may appear similar to benign lesions. The challenge is to distinguish malignancy.

LENTIGO maligna (LM) has a very different appearance to other melanoma subtypes. 

Despite the accepted wisdom that LM is mainly a lesion of the head and neck, it can occur at any site that has chronic sun exposure and so can occur on the limbs and trunk as well (see Figures 2 and 3). 

LM is most commonly seen as an in situ lesion, but especially in warmer parts of Australia, progression to invasive disease, called lentigo maligna melanoma (LMM), may be far more common than the literature suggests. 

LMM would appear to carry the same prognosis as that for other melanoma subtypes. In some parts of the world LM is becoming the most common form of melanoma.

The dermatoscopic features of facial LM are well described2 but not so of those occurring at other sites. This can make diagnosis of non-facial LM particularly difficult.

The histology of LM usually shows a single-layer proliferation of atypical melanocytes at the dermoepidermal junction, which produces an unexciting light brown colour with dermatoscopy, and usually not the “typical” dermatoscopic features of melanoma such as pseudopods, irregular black dots and clods, or white lines.

This situation is made even more difficult by the fact that invasive components of LM are commonly unpigmented. In fact, the dermatoscopy of LM more closely resembles that of solar lentigo (SL) than it resembles other melanoma subtypes. 

The challenge then is to distinguish the malignant LM from the benign solar lentigo. 

Clinically, LM is usually found against a background of multiple solar lentigos. It is distinguished by one or more of large size, colour variegation, and poorly demarcated border.(see Figure 4).

Most solar lentigos have a sharply demarcated “scalloped” border. A light brown lesion with any combination of curved and/or reticular lines and/or structureless zones and a well-demarcated scalloped border is likely to be a solar lentigo. Similar lesions but with poorly demarcated borders and Chaos should be carefully assessed for melanoma clues.

Dermatoscopic grey structures are a clue to malignancy in the Chaos and Clues algorithm, and although this also applies to lentigo maligna, it is less specific and will occasionally also lead to the biopsy of some benign regressing solar lentigos (see Figure 5).

Other useful clues such as grey circles and large polygons are less well described and will be the subject of a subsequent article.

Figure 1: Flowchart for the Chaos and Clues algorithm (modified by Dr Jeff Keir). Chaos is defined as asymmetry of structure or colour.

Figure 2: The natural habitat of lentigo maligna melanoma is on skin with marked solar damage. Multiple solar lentigos are expected, and previous non-melanoma skin cancer is the norm.

Figure 3: Macro of melanoma from Figure 2. This is an invasive lentigo maligna melanoma. The unpigmented area represents an area of invasive disease.

Figure 4: Two lentigo maligna on the upper arm. The inferomedial lesion is notable for its poorly demarcated border, the superolateral lesion for variegation.

Figure 5: Dermatoscopy of lentigo maligna resembles that of solar lentigo with curved lines being the prominent pattern. However, a poorly demarcated border and colour variegation (including the colour grey) are clues to assess this lesion more closely.

Dr Alan Cameron MBBS, FSccanz

Lecturer at the School of Medicine, The University of Queensland

Co-author: Dr Jeff Keir MBBS FSCCANZ