Introduction to the top-down 2-step approach
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The top-down 2-step pattern analysis approach builds upon the previous classic  and revised  2-step approaches by eliminating the requirement to differentiate melanocytic from non-melanocytic lesions in step 1. This algorithm hinges on the concept that the observer’s diagnostic accuracy for skin cancer, specifically melanoma, is, metaphorically, like a two-sided coin. One side of the coin requires the observer to make a specific diagnosis by recognizing the classic patterns/structures associated with nevi, dermatofibromas (DF), intradermal nevi (IDN), basal cell carcinomas (BCC), squamous cell carcinomas (SCC), lentigines & seborrheic keratoses (SK), angiomas, angiokeratomas, sebaceous hyperplasias, and clear cell acanthoma (CCA). Needless to say, the individual dermoscopic structures present in a lesion, within each diagnostic category (nevus, DF, BCC, etc), need to be placed within the context of the other features within the lesion. In other words, the global pattern defining a specific diagnosis is defined by the presence of distinct structures that have previously been found to carry a high predictive value for that specific diagnosis.
The other side of the coin requires the observer to acknowledge the nevus patterns that require context for their interpretation and the patterns and structures associated with melanoma. Armed with a clinical differential diagnosis followed by evaluation of the lesion via this top-down 2-step approach can facilitate the rendering of an accurate diagnosis or at least guide the clinician towards the most appropriate management plan.
In a simplified way, the main queries of the top-down 2-step analysis are: Step-1: To establish a specific diagnosis (if possible) Step-2: To rule out melanoma
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- Marghoob & Braun: Proposal for a revised 2-step algorithm for the classification of lesions of the skin using dermoscopy. Arch Dermatol 2010;146:426-8. PMID: 20404234. DOI.