Supplementary Material for: A case report of comparative approach in patient with multiple seborrheic keratosis: does it work for SK-like melanoma screening ?

Introduction : The evaluation of pigmented lesions remains a daily diagnostic challenge. Dermoscopy enhances accuracy but seborrheic keratoses (SK), though benign, may mimic melanoma due to melanocytic-like features. In patients with multiple SK, detecting a malignant lesion can be difficult, especially when it shares clinical or dermoscopic characteristics with SK.
Case Presentation : A 47-year-old man presented with multiple pigmented lesions on the face, scalp, and trunk. Most lesions were typical SK. However, one scalp lesion appeared clinically distinct with heterogeneous pigmentation. Dermoscopy revealed SK features combined with melanocytic criteria: a central bluish blotch and irregular globules. This "ugly duckling" lesion was excised and diagnosed as melanoma in situ, with no SK component histologically.
Conclusion : Seborrheic keratosis is a common benign tumor that can mimic melanoma, complicating diagnosis. The originality of this case lies in applying the ugly duckling sign—typically used to assess nevi—to multiple SK lesions. This comparative approach enabled the identification of a single atypical lesion, ultimately diagnosed as melanoma in situ. In patients with numerous SK, extending the use of this method beyond nevi may help detect malignant lesions that would otherwise remain unnoticed.