What is lentigo maligna?
Lentigo maligna is a type of melanoma in situ and is a precursor to lentigo maligna melanoma, a potentially serious form of skin cancer. Lentigo maligna is also known as Hutchinson melanotic freckle.
Lentigo maligna occurs in sun-damaged skin so is generally found on the bald scalp, face or neck, particularly the nose and cheek of older adults. It grows slowly in diameter over 5–20 years or longer.
What are the clinical features of lentigo maligna?
Lentigo maligna presents as a slowly growing or changing patch of discoloured skin. At first, it often resembles a freckle or benign lentigo. It becomes more distinctive and atypical in time, often growing to several centimetres over several years or even decades. Like other flat forms of melanoma, it can be recognised by the ABCDE rule: Asymmetry, Border irregularity, Colour variation, large Diameter and Evolving.
The clinical characteristics of lentigo maligna include:
- Large size: > 6 mm and often several centimetres in diameter at diagnosis
- An irregular shape
- Variable pigmentation — colours may include light brown or tan, dark brown, pink, red or white
- A smooth surface.
What is lentigo maligna melanoma?
Lentigo maligna melanoma is diagnosed when the malignant melanoma cells have invaded into the dermis and deeper layers of skin. Lentigo maligna has a comparatively low rate of transformation to invasive melanoma (under 5%). The risk of invasive melanoma is greater in larger lesions, with up to 50% of those with a diameter of greater than 4 cm being reported to have an invasive focus.
What are the clinical features of lentigo maligna melanoma?
Invasive melanoma is reported to arise within lentigo maligna in 3–10% of cases. The following features are very suspicious of invasion:
- Thickening of part of the lesion
- An increasing number of colours, especially blue or black
- Ulceration or bleeding
- Itching or stinging.
What are the dermoscopic features of lentigo maligna and lentigo maligna melanoma?
Dermoscopic features of lentigo maligna
- Pseudonetwork on the face: pigmentation arising around prominent facial hair follicles
- Asymmetrical pigmented follicular openings
- Rhomboid structures
- Grey dots/clods (these are the only diagnostic feature in some cases of lentigo maligna)
- Irregular structure
- Variation in colour
- Annular-granular pattern
- Grey circles
- Concentric circles (circles within circles)
- Structureless areas.
Dermoscopic features of lentigo maligna melanoma
Lentigo maligna melanoma shows the dermoscopic features of lentigo maligna and of invasive melanoma.
In the absence of a pigment network, there may be amelanotic areas and irregular blotches.
Annular granular pattern and rhomboids seen in dermoscopy of lentigo maligna melanoma Pseudonetwork, structureless areas, grey and concentric circles seen in dermoscopy of lentigo maligna melanoma Perpendicular white lines and an irregular blotch with streaking seen in dermoscopy of lentigo maligna melanoma, Breslow 0.95 mm
Annular granular pattern and rhomboids seen in dermoscopy of lentigo maligna melanoma
Pseudonetwork, structureless areas, grey and concentric circles seen in dermoscopy of lentigo maligna melanoma
Perpendicular white lines and an irregular blotch with streaking seen in dermoscopy of lentigo maligna melanoma, Breslow 0.95 mm
What is the dermoscopic differential diagnosis of lentigo maligna and lentigo maligna melanoma?
The dermoscopic differential diagnosis of lentigo maligna and lentigo maligna melanoma includes:
- Seborrhoeic keratosis
- Solar lentigo
- Lichen planus-like keratosis (LPLK), regressing lesion. Grey dots tend to be coarser and more evenly distributed in LPLK and finer and more irregular in lentigo maligna
- Pigmented actinic keratosis
- Melanocytic naevus
- Superficial spreading melanoma and other subtypes of melanoma.
What is the histological explanation for the dermoscopic features of lentigo maligna and lentigo maligna melanoma?
Lentigo maligna is a proliferation of malignant melanocytes along the basal layer of the epidermis and within the hair follicle.
Grey dots are formed by melanin in melanophages in the papillary dermis.
Asymmetrical pigmented follicular openings
Asymmetrical pigmented follicular openings are formed by melanophages within the hair follicle. Atypical melanocytes as single units or small nests extend along the circumference of the follicular epithelium.
Rhomboid structures are created by the merging of asymmetrical pigmented follicular openings.
Structureless areas are caused by the obliteration of the follicular openings.
Grey circles correspond to pigmented melanocytes within follicular infundibula. When melanophages are prominent, circles may appear to be comprised of grey dots.
Management of melanoma is evolving. For up to date recommendations, refer to Australian Cancer Council Clinical practice guidelines for the diagnosis and management of melanoma.