Fine needle aspiration cytology of dermatopathic lymphadenitis

OBJECTIVE: Dermatopathic lymphadenitis (DL) can be a nonneoplastic cause of an enlarged lymph node or nodes. Cytologic findings of DL have not bee well described. The differential diagnosis includes Langerhans nosis includes Langerhans histiocytosis (LH) in children and low grade lymphoma in adults. We present three cases of dermatopathic lymphadenitis with a discussion of cytologic findings and differential diagnosis. STUDY DESIGN: fine needle aspiration (FNA) findings of three cases of lymph nodes involved by DL were reviewed. All three were axillary lymph nodes in patients with skin rashes. Immunoperoxidase stains for S-100 and CD1a were performed on a cell block from one case, and flow cytometric analysis was performed on another. RESULTS: FNA of DL yields cellular smear with abundant histiocytoid cells with moderate cytoplasm and ovoid, vesicular nuclei with longitudinal grooves (interdigitating reticulum cells). These histiocytoid cells are S-100 and CD1a positive. The background cells are mature lymphocytes, scattered eosinophils and plasma cells. Case 1 was originally misinterpreted as consistent with LH. In case 2, flow cytometric analysis of a mixed population of lymphocytes was seen, ruling out lymphoma. In case 3, the diagnosis of DL was based on morphologic features. CONCLUSION: Interdigitating reticulum cells in dermatopathic lymphadenitis are S-100 and CD1a positive and could be confused with LH in children. In adults, flow cytometry could be used to distinguish them from low grade lymphoma. FNA findings of DL should be interpreted with the clinical history.