Beware black at the border

When using a dermatoscope, look very carefully at the edge of lesions.

Dermatoscopic black is produced by melanin at the surface of the stratum corneum. 

Melanin is a very efficient pigment and it absorbs all light reaching it. When there are no structures (such as epidermal, dermal cells or particles) between it and the observer to reflect or scatter incident light, it will appear black (Figure 2).

Malignant cells spread outwards early in the growth of melanomas (radial growth phase) and upwards towards the surface of the skin which is bounded by the stratum corneum. This is known as pagetoid spread. 

Peripheral black dots and clods are generally produced by melanoma cells or nests respectively, close to or at the level of the stratum corneum due to such pagetoid spread.

Central black structures frequently occur in naevi but when they are peripheral and asymmetrical this reflects disordered growth and is a clue to malignancy. 

Black structures do occasionally occur in pigmented basal cell carcinomas (pBCC) but rarely occur in pigmented squamous cell carcinomas (pSCC). 

Practice point

Peripheral black clods and dots are a strong clue to malignancy even if very small.

With a small lesion you have a lower threshold for assessing chaos. The small peripheral black structure may be the only feature causing the lesion to be asymmetrical.

Case study

A 40-year-old woman presented with a dark pigmented macule on her foot, 4 mm in diameter (Figure 4A). It had appeared and grown over six months. This lesion could be malignant (a melanoma, pBCC or pSCC) or benign (a solar lentigo/seborrhoeic keratosis or dermatofibroma). A new naevus would not be expected at age 40. 

Dermatoscopically (Figure 4B) the peripheral black clod (arrow) provided both the chaos (asymmetry) and clue (peripheral black clod). The presence of reticular lines ruled out both pBCC and pSCC, so the only diagnosis left was melanoma.

Dr Alan Cameron MBBS, FSccanz

Lecturer at the School of Medicine, The University of Queensland

Co-author: Dr Cliff Rosendahl

MBBS, FSccanz

Lecturer at the School of Medicine, The University of Queensland