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Most dermatologists diagnose melanoma by means of a global cognitive assessment that includes the ABCD algorithm, search for a mole that seems different from others (“the ugly duckling”), assessment of the number and types of moles, family and personal history of melanoma, biopsied nevi, and dermoscopy. The search for ugly duckling moles recognizes that each individual tends to “make moles” in a certain way. Although nevi are highly polymorphic in the general population, in a given individual many nevi share similar features. How does the brain discern the quacking nevus? These researchers hypothesized that humans have the ability to observe nevi appearance in a given individual, group them into “perceived similarity clusters” (PSCs), and detect dominant patterns.
To test this idea, the researchers had 18 observers including 9 dermatologists (“expert group”) and 9 observer novices (“nonexpert group”) examine images of nevi in a series of patients, one patient at a time. The observers were asked to cluster the nevi of each individual into PSCs using an ad hoc process designed to replicate actual skin examination. As predicted, the diversity of nevi in any given patient was low and limited to a few dominant patterns, regardless of the number of nevi. Experts identified a median of two or three PSCs in each person. In half of patients, the experts grouped 85% of nevi into no more than two clusters. About 20% of patients had a single cluster, and the remaining 30% had more variable nevi and a correspondingly larger numbers of PSCs. Novices could group moles, too, but not with the same level of concordance as experts. Dermoscopy did not produce greater intra-observer agreement.
Wazaefi Y et al. Evidence of a limited intra-individual diversity of nevi: Intuitive perception of dominant clusters is a crucial step in analysis of nevi by dermatologists. J Invest Dermatol 2013 Oct; 133:2355. (http://dx.doi.org/10.1038/jid.2013.183)
Comment
An ugly duckling mole isn't so worrisome when other, similar ugly duckling moles are quacking on the same patient. Most people make moles in a couple of ways, corresponding to placement in more than one perceived similarity cluster. People, especially trained people, can use a holistic cognitive assessment to group these in a global way not obviously reducible to color, color distribution, shape, border, or symmetry. This process may frustrate computer diagnosis of melanoma based on morphology of a single mole. That we can identify suspicious moles by recognizing outlier morphologies does not reduce the usefulness of dermoscopy. Dermoscopy reveals many less obvious features of nevi and likely adds specificity to the diagnosis of malignant melanoma.