The challenge of pigmented lesions

By Professor Rodney Sinclair and Dr Deepani Rathnayake

Case
A 69-year-old man with a past history of non-melanoma skin cancers presents with a slowly enlarging pigmented macule on the right labial commissure, which has been present for one year (figure 1 above right).

The lesion has recurred after three sessions of liquid nitrogen cryotherapy. Clinically, the lesion looks to be a flat, pigmented, seborrheic wart.

Investigations
Dermoscopy is inconclusive. A 6mm × 4mm shave biopsy was performed from the darkest area seen on dermoscopy (figure 2 below).

The histology was reported as an early, benign, lentigo/pigmented seborrheic keratosis, with no evidence of malignancy.

As the histology had identified a melanocytic component, the patient is referred to a plastic surgeon for surgical excision of the entire lesion, with a narrow margin, and closure with a full-thickness skin graft.

The histology from the excision specimen was reported as melanoma in situ (lentigo maligna) with no invasive melanoma. The scar was re-excised with a 5mm margin and closed with a new skin graft (figure 3 below).


Figure 2: A shave biopsy was performed on the darkest area seen on dermoscopy.


Figure 3: The patient was referred to a plastic surgeon for surgical excision.

Differential diagnoses
Flat, pigmented lesions on the face are a challenge to diagnose clinically. The key differential diagnoses are seborrheic keratosis, pigmented actinic keratosis, solar lentigo and lentigo maligna.

Dermoscopy may increase the diagnostic accuracy if typical features of seborrheic keratosis are present.

The dermoscopy features of lentigo maligna, such as rhomboidal structures, annular–granular structures and grey pseudo-network, are often absent in early lesions.1

The optimal biopsy is an excisional biopsy with a 2mm margin. Large lesions may require a ‘flap of skin’ graft to achieve wound closure, which is not always technically possible.

In some circumstances, a shave biopsy or punch biopsy may be performed for diagnosis and surgical planning. However, shave biopsy or punch biopsy cannot be used to exclude the diagnosis of lentigo maligna.

There is a recognised risk of sampling error with incisional biopsy that is increased in patients who have had previous therapeutic interventions, such as cryotherapy.

Diagnosis
The clinical differential diagnosis in this case was either seborrheic keratosis or lentigo maligna. The lesion was too big to be a simple solar lentigo.

The presence of a melanocytic proliferation (solar lentigo) in the initial histology excluded the diagnosis of simple seborrheic keratosis and mandated an excisional biopsy.

The histological diagnosis of solar lentigo on the initial shave biopsy is required to be considered with reservation in this clinical situation, and does not negate the requirement to proceed to excisional biopsy.

Shave biopsy or punch biopsies of facial pigmented lesions may be performed to establish a diagnosis of lentigo maligna, but cannot be relied upon to exclude the diagnosis, and in particular should not be relied upon to distinguish between solar lentigo and lentigo maligna.

Discussion
Lentigo maligna is also known as Hutchinson melanotic macule (freckle) and melanoma in situ. It is a melanoma of the epidermis. Lentigo maligna does not metastasise.

If dermal invasion of melanoma occurs, it then becomes lentigo maligna melanoma.

The risk of metastasis from lentigo maligna melanoma is the same as for superficial spreading melanoma. Lentigo maligna is most common in the elderly, and in sun-damaged skin on the face and neck.

It usually grows slowly over many years. Lentigo maligna has a 3-5% chance of developing into invasive melanoma, with the risk increasing with the size of the lesion.2

Outcome
The patient’s lentigo maligna was ultimately fully excised, with the graft healing well. He requires regular review as his sun-damaged skin — along with his past history of non-melanoma skin cancers and lentigo maligna — places him at greater risk of malignancies.

Professor Sinclair is head of dermatology at Sinclair Dermatology, Melbourne, Victoria; and director of Epworth Dermatology, Melbourne, Victoria. Dr Rathnayake is a dermatologist at Sinclair Dermatology, Melbourne, Victoria.

References

  1. Schiffner R, et al. Improvement of early recognition of lentigo maligna using dermatoscopy. Journal of the American Academy of Dermatology 2000; 42:25-32.

  2. Weinstock M, Sober A. The risk of progression of lentigo maligna to lentigo maligna melanoma. British Journal of Dermatology 1987; 116:303. 

This article first appeared in Australian Doctor 7th of October, 2014

Case A 69-year-old man with a past history of...

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