New dark mole on arm-how to analyse it

Always be cautious about a new mole, especially over the age of 30. Skin cancers are very rare under the age of 20, become steadily more common with each passing year, and really take off after age 70. But a new mole (*) always deserves some thought and ideally the opinion of a doctor, nurse or other health worker trained to use a dermatoscope.

As the above chart shows, melanoma skin cancer incidence increases steadily after the age of 20.

Here is an example of a new mole that stands out as different.

A new stand-out solitary pigmented lesion like this deserves attention. The upper arm is often exposed to sun and is one of the more common locations to find a melanoma or other skin cancer. The lesion is not the most dramatic you can imagine, but the borders seem somewhat angular and the shades of brown variable. If we use the simple ABCD rule, this pigmented lesion is somewhat asymmetrical, border is irregular, colour is variable and diameter over 6mm, so that is a concern although not proof.

The dermatoscopic view reveals additional data. I will discuss these findings from 2 points of view, firstly the two-step algorithm, and next chaos and clues (pattern analysis).

The two step algorithm asks two questions:

1. Is this lesion melanocytic-yes or no?
2. If it is melanocytic, could it be a melanoma? (or in other words, can I confidently say it is benign?)

Now the thoughtful person will immediately ask ‘And how can we tell if it melanocytic or not?‘ Good question. This is a weakness of the algorithm, because we cannot always accurately make that judgment. However, there are some rules we can apply that will usually give us the right answer.

How to decide if a lesion is, or is not, melanocytic.

Firstly, if it is something else, a lesion based on another cell type, it can’t be melanocytic. With training and experience, we can often make a positive dermoscopic ID for a wart, a haemangioma or a dermatofibroma. If the lesion is not melanocytic, it cannot be a melanoma.

Next, many if not most melanocytic naevi (at least the flat ones) have a reticular pigment network, which is easily seen on dermoscopy. Others, especially congenital or in people under 20, have a globular (or clod) pattern.

Caution: some other lesion types may have reticular or globular patterns on dermoscopy, but not usually as clear cut as when seen in naevi. You need to study many examples to get the feel of it-which is why I post examples and links to other free learning resources for the benefit of learners.

Thirdly, some naevi have multicomponent or completely featureless patterns. Such lesions are reckoned to be melanocytic by default-so, for the purposes of the two-step algorithm, if you can’t prove a lesion is non-melanocytic, assume it is melanocytic.

STEP TWO If melanocytic, could it be a melanoma? This is basically to ask the question, ‘Can I confidently reassure the patient that this mole is harmless?’ How do we do that? Essentially, by symmetry of colour and structure, and the absence of any of the 9 dermoscopic clues to malignancy. And the more I reflect and write about the two-step algorithm, the more I am reminded of why it and other algorithms have tended to be replaced by the chaos and clues approach!

I call chaos and clues ‘The one algorithm to rule them all.’ as it is in effect distilled from all the good bits of all previous algorithms.

Anyhow, lets do a 2-step on this lesion.

1. is it melanocytic?

I can’t see reticular network, or clods/globules, so that’s no help. Can I make this one of the 4 common benign non-melanocytic lesions? Well, it certainly looks nothing like a dermatofibroma, haemangioma, wart or sebaceous gland hyperplasia. It is however multicomponent and mostly featureless (amorphous, lacking overall structure). So, by default we must label this lesion as melanocytic. So, question two,

2) Could it be a melanoma?

As a matter of first principle, we scan back from the dermoscopy and consider the history and naked eye appearance. Always take the holistic approach-as the great Harald Kittler has said ‘Dermatoscopy alone is nothing.’ (of course Professor Kittler was exaggerating for effect, you often can make a good diagnosis on dermoscopy alone, the AI machines prove that, but the point about history and context needs making).

It’s a new, stand-out lesion on sun exposed older white skin. That’s enough to make us concerned.

Next, looking at the dermoscopy, our eyes are drawn to a blue featureless area. Next to this there is a splash of white. That’s 2 clues for melanoma. This is plenty enough evidence for excision biopsy.

Now let’s do a Chaos and Clues on the same lesion. Let’s take the history and plain view as read and go straight to the dermoscopy. It is chaotic? Yes, because we see multiple colours and asymmetry. Are there any clues to malignancy? Yes, the blue featureless and the shiny white structure. So we excise

So, the Chaos and Clues and the Two-Step algorithms have led us to the same (correct) decision, but the chaos and clues did it quicker and by a simpler process.

HISTOLOGY was a melanoma, 0.4mm thick, so 99.5% likely to be cured. Catching them early saves lives, which I why I have just spent around 2 hours creating this tutorial in my own time without pay.

Professor Kittler, who with Professor Cliff Rosendahl originally created the Chaos and Clues algorithm, (**) has said ‘The expert learns all the algorithms and then designs his own.’

Professor Rosendahl and Amanda Oakley (founder of the great Dermnetnz educational web site) will be visiting Manchester, England, next 28/29 August for a 2 day skin cancer diagnostics course. The following day August 30th there will be a skin lesion diagnostics Masterclass with international expert Professor Bengu Nisa Akay from Ankara, Turkiye and Ausama Atwan from Newport, South Wales. This will be brilliant for any health care worker, NHS or private, whose work involves evaluating skin lesions. Don’t miss the super early bird price!

(*) The term ‘mole’ when used by doctors will usually mean a melanocytic naevus. The general public will often use the term to refer to any sort of skin lesion. The distinction is important, as any melanocytic lesion could in theory develop into (or already be) a melanoma skin cancer, whereas non-melanocyte skin lesions have no such potential.

(**) See their brilliant book: Dermatoscopy: Pattern Analysis of Pigmented and Non-Pigmented Skin Lesions’.