Isolated nervous system involvement on initial presentation with histiocytic neoplasms occurs in up to 5% of patients, according to results published in Blood Cancer Journal. Ronald S. Go, MD, and colleagues examined the frequency of isolated nervous system involvement at initial evaluation for histiocytic neoplasms and eventual development of systemic involvement in 377 adults. Isolated nervous system involvement was seen in 19 patients (5%), including six with Langerhans cell histiocytosis, seven with Erdheim-Chester disease (ECD), and six with Rosai-Dorfman disease. Frequent neurological symptoms and signs included headache and weakness. Most patients had isolated nervous system involvement on systemic imaging after diagnosis, but two patients with ECD developed systemic involvement less than 12 months after presenting with isolated involvement. “This series raises the possibility of a progenitor cell line arising in the CNS, rather than in the systemic myeloid system,” Dr. Go and colleagues wrote.