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Background: Rosai-Dorfman disease (RDD) is a rare benign idiopathic histiocytic proliferation. Most commonly reported cases are lymph nodal. Only 30-40% of cases occur in extranodal sites. Although the morphological features of RDD are well documented, there is limited information about the morphometric variations in the histiocytes of this entity. Method: Twenty-two cases of RDD diagnosed on fine-needle aspiration cytology (FNAC) were retrieved from the archives. Both Papanicolaou- and May-Grünwald-Giemsa-stained slides were available for evaluation in all cases. Nuclear area, diameter and histiocyte size were measured taking reactive histiocytes as controls. Results: Among the 22 patients (male/female ratio 3:2; age range 5-55 years, mean 26 years), 3 cases were extranodal and 19 cases were nodal. The nodal sites included cervical, axillary, inguinal and submandibular lymph nodes. The extranodal sites were the retroperitoneum, mediastinum and skin. The most common clinical presentation was enlarged lymph nodes. Cytological features included numerous large benign histiocytes with emperipolesis. All the morphometric parameters were significantly (p < 0.01) higher in RDD histiocytes than in histiocytes in the reactive lymph node. Conclusions: In view of the rarity of the disease and the variable clinical presentation in RDD, accurate diagnosis is necessary. This is the first study to document the morphometric parameters of RDD histiocytes and their comparison with histiocytes in the reactive lymph node.

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