[Primary cutaneous Rosai-Dorfman disease: a diagnostic challenge].

Rosai-Dorfman disease (RDD) is a rare histiocytic disorder characterized by the proliferation of S100-positive, CD1a-negative histiocytes. Although it classically presents with massive lymphadenopathy, extranodal involvement may occur, including the skin. Primary cutaneous RDD, without nodal or systemic involvement, is particularly uncommon and poses a diagnostic challenge due to its nonspecific clinical presentation. We report the case of a 39-year-old male patient presenting with a solitary, asymptomatic erythematous nodular lesion on the upper back, with progressive growth. Initial imaging studies suggested a benign cystic lesion. Dermoscopic examination revealed yellow-orange structureless areas associated with atypical vascular patterns. The definitive diagnosis was established by excisional biopsy, which demonstrated a dermal histiocytic infiltrate positive for S100 and CD68 and negative for CD1a, consistent with Rosai-Dorfman disease. Systemic evaluation showed no evidence of extracutaneous involvement, confirming the diagnosis of primary cutaneous RDD. This case highlights the need to include cutaneous RDD in the differential diagnosis of atypical nodular skin lesions and emphasizes the importance of clinicopathological correlation for accurate diagnosis.