ETMR-31. A UNIQUE CASE OF CNS DIFFUSE, NON-SYSTEMIC ALK+ HISTIOCYTOSIS IN A PEDIATRIC PATIENT

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Abstract BACKGROUND Histiocytic disorders are a group of rare diseases with accumulation of of macrophages, monocytes, or dendritic cells in various parts of the body. ALK-positive histiocytosis is a rare subtype of histiocytic neoplasm first described in 2008 that is very rarely described in pediatrics. METHODS A 20 month old male presented to an adult hospital after a prolonged period of headaches, vomiting, and loss of developmental milestones. Imaging demonstrated severe hydrocephalus and extensive thickened leptomeningeal enhacement with mass effect. He underwent VP shunt placement and revision, a T7-T10 laminectomy, and tumor debulking. Extensive molecular testing, including DNA and RNA analysis by NGS was performed and a CLTC::ALK fusion was identified, along with Tier III DNA variants, yielding a diagnosis consistent with ALK-Positive Histiocytosis, with diffusivity in the CNS system. Full evaluation demonstrated no involvement of the peripheral nervous system or other systemic involvement. He initially demonstrated signs of neuro-irritation, altered tone, and lower extremity clonus. RESULTS After review of very little literature published on similar patients, although none with a clinical picture matching his, he was started on a treatment course of high dose dexamethasone, vinblastine, and lorlatinib targeting the ALK+, likely driver, mutation of his disease. On evaluation scans two months into treatment, he showed no change in his disease burden. He was transitioned to clofarabine cycles with daily lorlatinib for continued targetted therapy. After six total cycles, he demonstrated a high partial to complete remission (allowing for evidence of scarred tissue) and tremendous neurologic and developmental milestone gains. CONCLUSIONS This case demonstrates a unique presentation in which there is little described in the literature in way of clinical features or treatment standards, and further none exactly similar to this patient. After undergoing extensive evaluation and therapy modication, he responded remarkably well and continues to have optimal quality of life.