Castleman-Like Lymphadenopathy in a Patient with Mixed Connective Tissue Disease: A Case Report and Review of the Literature.

Differentiating reactive lymphadenopathies in the context of autoimmune disease from Idiopathic Multicentric Castleman Disease (iMCD) poses a significant diagnostic challenge. Castleman-like histological features have been described in various autoimmune disorders, necessitating a strict and comprehensive integration of clinical and laboratory findings to reach the correct diagnosis. Although the Castleman Disease Collaborative Network (CDCN) consensus guidelines list several autoimmune conditions as exclusion criteria for an iMCD diagnosis, mixed connective tissue disease (MCTD) is not currently among them. We report the case of a  77-year-old woman presenting with fatigue, Raynaud's  phenomenon, sclerodactyly, mild generalized lymphadenopathy, in whom the lymph node biopsy revealed a Castleman-like histology. The absence of systemic inflammatory symptoms and the presence of high-titer anti-U1- RNP antibodies were, however, inconsistent with iMCD, favouring the diagnosis of a reactive Castleman-like lymphadenitis secondary to MCTD. This report highlights that Castleman-like lymphadenopathy can occur in MCTD, closely mimicking iMCD. Therefore, in patients with autoimmune diseases not explicitly listed among the CDCN exclusion criteria, comprehensive clinicopathological integration is essential to avoid misdiagnosis and potentially inappropriate antiIL-6-based therapy.