Lentigo maligna is a subtype of melanoma that arises on sun-damaged skin. The term lentigo maligna denotes melanoma in situ, whereas lentigo maligna melanoma (LMM) denotes invasive melanoma. We use LMM as a term that encompasses all melanomas (in situ and invasive) on sun-damaged skin. Melanoma on sun damaged skin (extrafacial) is covered in a separeate chapter LMMs occur most frequently in the elderly population. The common anatomic sites include the sun-damaged skin of the bald scalp and face.
LMMs are patches to flat plaques, variegate in colors that range from light brown to black, and asymmetric in shape. These lesions tend to enlarge slowly and if left untreated, cover an area of several centimetres while remaining flat, at times appearing discontinuous and patchy on clinical examination. To appreciate the extent of the clinical lesion, Wood’s lamp is an essential part of the examination, and often, it reveals that the lesion is much more extensive than it appears on naked-eye examination. With time, LMM may develop invasive foci clinically evident as papules, nodules, or thicker plaques. LMM at that stage confers risk of metastatic disease.
Histopathologically, findings in LMM include an asymmetric lesion with an abnormal proliferation of single melanocytes in the basal layer of the epidermis, the melanocytes being unevenly spaced and increased in density (crowded), and often extend down follicles. Even few nests of melanocytes in a broad, junctional lesion on the face that displays solar elastosis in the dermis (evidence of sun damage) strongly raise the suspicion of LMM. An irregular scatter of melanocytes in pagetoid fashion (at suprabasal layers of the epidermis) also supports the diagnosis of LMM. Nests and fascicles of melanocytes in the dermis attest that the melanoma is no longer in situ. LMM in the dermis may display at times desmoplastic and neurotropic features.
Early LMM may be clinically subtle and difficult to diagnose on the sundamaged background that displays many solar lentigines. Other differential diagnoses include pigmented actinic keratosis, lichen plaunus-like keratosis, and early, flat seborrheic keratosis or solar lentigo. On the face, a pigment network is usually absent due to the flattening of the dermoepidermal junction (DEJ) and effacement of the rete ridges.
Thus, additional dermoscopic criteria are applied to diagnose early LMM.
Classic dermoscopy criteria (Stolz Criteria) :
Hyperpigmented follicular openings
There is crescent-shaped pigmentation that accentuates in only one portion of the follicular opening. On histopathology, this finding correlates with atypical melanocytes as single units or small nests extending down hair follicles. Correlation of dermoscopically identified asymmetric follicular openings using refl ectance confocal microscopy (RCM) demonstrates a nonuniform infiltration of atypical melanocytes along the circumference of the follicular epithelium, explaining the asymmetric, crescent-shaped pigmentation seen en face by dermoscopy. On occasion one can see asymmetrically pigmented follicular openings in a solar lentigo; however, in a solar lentigo the color of the crescent-shaped area tends to be the same color as the surrounding area.
In LMM the follicular openings can be symmetric or asymmetric but the color tends to be darker, often with a grayish hue.
The following 5 pattern of hyperpigmented follicular opening can be observed:
- Fine circle
- Signet ring-like circle
- Irregular circle
- Double circle
Annular–granular pattern can be divided into dots aggregated around adnexal openings and short and polygonal lines around and between adnexal openings.
These two components of the annular–granular pattern can be seen singly or in combination.
Dots aggregated around adnexal openings:
The findings range from brown dots to blue-gray granularity scattered throughout the lesion, but often appearing to cluster around the adnexal openings (Fig. 8d.6). Based on dermoscopy to RCM and histopathology correlation, these findings are explained by aggregates of melanocytes and small nests at the DEJ between the follicles (brown dots) and by melanophages in the dermis (blue-gray granularity).
Short and polygonal lines around and between adnexal openings.
As the density of the brown dots increases, they coalesce to form short pigmented lines around the adnexal openings as well as between the adnexal openings. These lines are often polygonal, forming a zig-zag pattern.
Dermoscopy to RCM and histopathology correlation suggest that these structures are due to confluent junctional nests and aggregates of melanocytes by the annular–granular pattern, the concentric dermoscopic appearance has been termed isobar (also known as circle within a circle).
Pigmented rhomboidal structures
Elongation, thickening, and merging of the short polygonal lines around adnexal openings form a polyhedral-shaped structures that have been termed “rhomboidal” (in reality, the shapes vary but are all polygonal). The histopathologic correlation is similar to that of short and polygonal lines, albeit reflecting more extensive infiltration of the DEJ by confluent nests and aggregates of melanocytes.
Dark blotches and obliterated hair follicles
Dark blotches (also termed “homogenous structures”) with or without obliteration of the adnexal openings. Dark brown to black blotches are seen, initially with “sparing” of adnexal openings, appearing as light-colored holes within the blotch , and eventually, as a homogenous black blotch with obliteration of the adnexal openings. It should be highlighted that the aforementioned melanoma-specific structures can be seen in lentigo maligna melanomas on the face and also in lentigo maligna melanomas arising on nonfacial chronic sun-damaged skin.
In this lentigo maligna melanoma the double circle (circle in circle) pattern is easy to see. There is also obliteration of adnexal openings:
New dermoscopy criteria (Thomas criteria) :
Darkening at dermoscopic examination
This defines the observation on dermoscopic images of the presence of a colour, invisible to the naked eye, and darker than all clinically observable shades of brown or grey. Target-like pattern We defined as ‘target-like pattern’ the presence of a dark dot in the centre of the dark circle of a hyperpigmented hair follicle. This dark dot was not a hair.
Red rhomboidal structure
We defined as red rhomboidal structure a lozenge-shaped vascular pattern occurring in the area separating the hair follicles from the others.
Increased density of the vascular network
‘Increased density of the vascular network’ was defined as a vascular network of higher density within the lesion than in peripheral skin.
Management of pigmented lesions of the face
The first issue that needs to be addressed when evaluating a pigmented lesion on the face with dermoscopy is whether the lesion possesses any melanoma-specific structures. Although superficial spreading melanoma is rare on the face, they can occur and thus all facial lesions need to be scrutinized for the presence of any melanoma-specific structures.
If the lesion has any of the aforementioned structures then the lesion needs to be biopsied. If it does not have any of the above-mentioned melanoma-specific structures then the lesion is evaluated for the presence of features diagnostic of basal cell carcinoma, solar lentigo, or seborrheic keratosis. If the lesion has no features to assist in diagnosis then the lesion should either be biopsied or subjected to digital monitoring for at least 6 months to one year.
Based on: Atlas of Dermoscopy by Marghoob A., Malvehy J., Braun