Very irregular but harmless pigmented skin lesion

The ABCD rule (Asymmetry, Border irregular, Colours mixed, Diameter over 6mm) is a useful rule of thumb for evaluating suspicious skin lesions. It is true to say that most melanomas will fail ABCD. However, the rule used alone (without dermoscopy) will miss small, featureless and non-pigmented melanomas, and some of these can be the most dangerous ones. It will also pick up many harmless skin lesions as ‘false positive’ melanomas, and, unless dermoscopy is used to clarify matters, this can lead to needless alarm and unnecessary surgery.

Here is a typical example of a pigmented skin lesion which fails ABCD but is harmless.

Here we see 7 or 8 light to medium brown skin lesions on older white skin. I will add that they are all flat and non-scaly to touch (*). One of the lesions is much larger than all the others and has a very irregular shape. It qualifies as an ‘Ugly Duckling’ and violates the ABCD rule by being Asymmetrical, Border irregular and Diameter well over 6mm (in fact around 15mm diameter). Does this mean we should cut it out? Not without dermoscopy!

Dermoscopy confirms the very irregular shape. However, experts agree that shape ALONE, although it gets our attention, is not proof of malignancy. The same goes for size. If we get over our initial impression of a very odd shape and study the consistency of the border, we see that although it is very wavy, it is crisp and even in character. You could trace around it easily-the edge is well defined, not wishy-washy and vague. If we have previously used our brain to study the natural history of common skin lesions (*) then we will remember that this even border, even if irregular in outline, is typical of solar lentigo-which is a very common and harmless skin lesion in older white, sun-exposed skin.

The lesions scores A for Asymmetry, B for irregular Border and D for Diameter over 6mm, so that’s 3 out of 4 on the ABCD rule. Colour is OK, I see only brown (mostly light brown, a little medium brown). As far as the dermoscopy is concerned, I call this featureless-there is no overall pattern and it is mostly very homogenous. We can see skin markings, the pale straight lines, and some authorities suggest these may be a weak clue to lentigo maligna, but many others, including me, disagree with that opinion. Certainly there are no other arguable features which are positive for lentigo maligna, which usually have multiple colours, angulated lines, grey or blue areas, inconsistent border, grey circles, granular dots etc.

This lesion was confidently diagnosed as a benign solar lentigo and the patient reassured and advised. I will put up some more examples of solar lentigo (plural lentigines) shortly.

NB the term ‘solar lentigo’ is potentially misleading, given that the word ‘lentigo’ is also used in ‘lentigo maligna’. The use of the word lentigo in both terms may suggest that a benign lentigo can turn into a malignant lentigo. This idea is completely mistaken. A solar lentigo is a pigmented lesion made of pigmented keratinocytes, whereas a lentigo maligna is made up of melanocytes, which are a completely different cell type (by all means click on the links for details). A keratinocyte can no more turn into a melanocyte than a fish can turn into a horse. It is therefore impossible for a solar lentigo to turn into a lentigo maligna. It IS of course possible for fallible humans like us to misdiagnose one for the other! Hopefully studying cases like this will reduce that risk.

NB The term lentigo or lentiginous refers to a particular arrangement of certain skin cells as seen under a microscope. I won’t try to explain what a histopathologist means when he or she says ‘lentiginous’ as it will just confuse matters. Ideally the terms should be changed. If it was up to me I would suggest abolishing the term lentigo maligna and replacing it with melanoma in situ which, as near as makes no difference, means the same thing.

(*) I teach the BEETS holistic diagnostic approach, which stands for

Brain-use your brain to learn all you can about the biology and behaviour of common and important skin lesions

Ears-listen to the patient telling you the story of this skin lesion, how long they have had it, how has it behaved, have they had a skin cancer or high risk occupation etc

Eyes– look at it, and ideally look at the rest of the skin as well for comparison

Touch-light palpation can tell you a lot

Scope-what you have studied skin lesions, listened to the patient, looked at and touched the skin, THEN use the dermoscope. Having done all this, put everything together for your diagnostic hypothesis, and then make a decision whether to reassure, review, refer or remove.