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Background: Preoperative diagnosis of malignant collision tumors (MCT) is extremely difficult. The value of dermoscopy to improve the correct detection of these tumors has not been previously studied. This study aims to evaluate the diagnostic accuracy of MCT with and without dermoscopy and to describe the dermoscopic features of a large series of MCT. Methods: Dermoscopic images of 161 MCT were evaluated. Clinical and dermoscopic images of histopathologically proven MCT intermingled with other tumors were randomly presented to clinicians with different levels of experience, blinded to the diagnosis and objective of the study. The clinical and dermoscopic diagnostic accuracies were measured separately. Results: A total of 161 histopathologically proven cases of MCT were collected. The most frequent MCT was basal cell carcinoma-seborrheic keratosis collision tumor (CT; 37.9%), followed by basal cell carcinoma-melanocytic nevus CT (19.9%), and melanoma-seborrheic keratosis CT (6.8%). Diagnostic accuracy among experts on dermoscopy was 71.4%. The study included 119 participants. The percentage of correct diagnoses was 8% by naked eye examination and 36.4% by dermoscopy (p < 0.001). The presence of the malignant component in the cases of MCT was not recognizable in 19.1% of cases by naked eye examination and in 11.8% of cases by dermoscopy (p < 0.001). Conclusions: The diagnosis of MCT can be assisted and clarified by dermoscopy. However, many of these lesions manifest complex morphologies and continue to be challenging, even for experts on dermoscopy. Atypical, uncertain, or non-classifiable lesions still need a complete excision for the final diagnosis. [ABSTRACT FROM AUTHOR]

Abstract: Background/Objectives: The dermoscopy of lymphangioma circumscriptum, also known as superficial lymphatic malformation, remains to be clarified. Methods: Digital dermoscopic images of 45 histopathologically confirmed cases of lymphangioma circumscriptum collected from nine hospitals in Spain, Italy and Turkey were evaluated for the presence of dermoscopic structures and patterns. Results: Our study shows that the most common structure found in lymphangioma circumscriptum was the presence of lacunae (89% of cases). The latter were red or dark‐coloured in 18 cases (45%), yellowish or whitish in 14 cases (35%) and multicoloured in eight cases (20%). The second most common dermoscopic structure was the presence of vascular structures, which were found in 82% of cases, followed by white lines (47%), the hypopyon sign or two‐tone lacunae (42%) and scales (7%). Conclusions: Dermoscopy is useful in improving the diagnosis of lymphangioma circumscriptum with characteristic structures and patterns and could assist in elucidating the presence of blood in lymphatic channels. [ABSTRACT FROM AUTHOR]