In their manuscript, Dermoscopy in Merkel cell carcinoma (MCC)—A Multicenter Retrospective Study of the International Dermoscopy Society (IDS) 1, the authors have taken on a very challenging task: attempting to define the dermatoscopic features of an extremely rare, non-pigmented, highly malignant tumor. To maximize the number of cases, the authors have harnessed the worldwide membership of the IDS, a group with more than 7500 current members. These members have submitted images of 134 cases of MCC, as well as 268 control lesions. MCC is very rare. For context, in a 2021 study of all melanomas diagnosed by 27 Australasian general practitioners in 2013 2, unpublished data reveal that there were, in addition to the 637 melanomas, only two cases of MCC, one of which was prospectively diagnosed correctly, the other having a provisional diagnosis of squamous cell carcinoma (SCC). Surprisingly, the same unpublished data from that study show that there were no other provisional diagnoses of MCC, suggesting that in 2013, it was not high on the list of considerations for pink/red, raised tumors among those GPs. In the almost four decades since dermatoscopy has become readily available, the dermatoscopic features of many diagnostic entities have been characterized. Pigmented skin lesions exhibit numerous highly specific clues, and many non-pigmented clues, such as white circles and white structureless areas in SCC 3 and stromal and vessel clues in basal cell carcinoma (BCC) 3, have been defined. Amelanotic melanomas are more challenging, leading to frequent delayed diagnosis 4, but relevant dermatoscopic clues have been characterized, including polymorphous vessels with patterns of both linear-irregular and dot vessels 3. While polarizing-specific white lines can often help distinguish malignant from benign non-pigmented lesions, they do not distinguish melanoma from BCC 3. The authors of the IDS study 1 have attempted to identify specific dermatoscopic clues for MCC, as exist for most common skin lesions, both benign and malignant. The study is well designed. It is much larger than any previous study on MCC, and it is the first that includes an analysis of control lesions, with these cases being submitted by contributors of cases of MCC, having been selected on the basis of appropriate pre-defined characteristics. These control lesions comprised 49 distinct diagnoses. The rating process was rigorous, without time constraints, and the statistical analysis was robust. The findings of the study are therefore valid, but—perhaps unsurprisingly given “the nature of the beast”—disappointing. Non-pigmented malignant nodules have relatively few features on which to base a diagnosis, and these features frequently overlap. The positive predictors of MCC, pink color and scales, will do nothing to help the clinician distinguish this lesion that they may encounter once or twice in a lifetime from many thousands of other non-pigmented lesions they will assess. No discriminatory features were identified with respect to vessel patterns or shiny white structures. The concluding statement that amelanotic melanoma should be included in the differential whenever MCC is suspected is very telling. Nothing identified in this study discriminates between those entities. Before we published the first dermatoscopic image of a MCC in 2009 5, its dermatoscopic features were unknown (Figure 1). That has changed, however; in spite of a large, well-designed and well-executed study, there do not appear to be even reasonably specific dermatoscopic clues to a diagnosis of MCC. Neither are there specific dermatoscopic features to discriminate MCC from other raised, non-pigmented lesions. For the time being, the clinical features of an abruptly appearing, rapidly growing, pink or red nodule (Figure 1) may continue to trump dermatoscopic features when those features lack specific clues to another diagnosis. Open access publishing facilitated by The University of Queensland, as part of the Wiley - The University of Queensland agreement via the Council of Australian University Librarians. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.
Rosendahl et al. (Mon,) studied this question.