A Rare Case of Erdheim-Chester Disease with Pseudoprogression in the CNS.

Erdheim-Chester Disease (ECD) is a rare histiocytic neoplasm characterized by multi-organ tissue infiltration by histiocytes. Clinical presentation and course can be heterogeneous, ranging from localized and asymptomatic bone lesions to a multisystem disease involving skin, cardiac, pulmonary, retroperitoneum, lymph node and central nervous system (CNS) with significant morbidity and mortality. Herein, we describe a rare case of a 61-year-old female patient with ECD with primarily brain and bone involvement who developed "progressive disease" in the CNS after 3 cycles of cladribine. The patient elected to forego any further treatment based on her clinical stability despite progressive disease imaging findings. Subsequent imaging while off treatment demonstrated marked interval improvement in the previously patchy areas of involvement and the patient has continued to have clinical stability 4 years after stopping therapy. The patient was deemed to have experienced pseudoprogression. Pseudoprogression of ECD has not been reported in the literature, and it may be associated with localized inflammatory cytokine release in response to treatment. The possibility of pseudoprogression needs to be considered in evaluating therapeutic response in ECD patients.