Rosai-Dorfman Disease With Novel BRAF Fusion Involving the Central Nervous System.

Rosai-Dorfman disease is an uncommon histiocytic neoplasm that is often self-limiting; however, rarely, it exhibits aggressive behavior and may undergo biological transformation to histiocytic sarcoma. Pathogenic genetic drivers of Rosai-Dorfman disease include recurrent genetic abnormalities in KRAS, NRAS, ARAF, and MAP2K1. Mutation of BRAF is a well-known oncogenic driver in histiocytic neoplasms, including up to 50% of Langerhans cell histiocytosis lesions, Erdheim-Chester disease, and adult and juvenile xanthogranuloma. However, BRAF mutations have been documented in only 4 reported Rosai-Dorfman disease specimens, and none were rearrangements. Herein, we report the first instance of Rosai-Dorfman disease with OSBPL11::BRAF gene rearrangement, a unique gene fusion, and describe its clinicopathological features.