A 33 year-old right handed man with Philadelphia-positive acute lymphoblastic leukemia initially diagnosed 09/2015, with two prior CNS relapses in 12/2016 (right facial weakness and headaches, resolved with intrathecal chemotherapy) and 03/2017 (recurrent right facial weakness, received intrathecal cytarabine, radiation to 17.5Gy in the skull base), craniospinal irradiation 05/2017, matched sibling donor stem cell transplant 06/12/17, presented in July 2019 with a three-week history of gait instability culminating in right facial paresthesias. His neurologic exam was notable for sensory loss in the first two divisions of the right trigeminal nerve with mildly wide-based station and gait and a tendency toward right lateropulsion. MRI brain demonstrated new contrast enhancing parenchymal lesions in the right pons/cerebellar peduncle/medulla amid surrounding T2/FLAIR hyperintensity suspicious for rhombencephalitis versus possible radiation necrosis. MR spectroscopy was notable for depression of all metabolites with no evidence for increased permeability or blood flow.  CSF was notable for lymphocytic pleiocytosis with 28 WBCs and normal protein. He received high-dose solu-medrol with no improvement--rather developing a complete facial palsy--then bevacizumab and finally Rituxan for a presumed diagnosis of post-transplant lymphoproliferative disorder versus autoimmune rhombencephalitis versus radiation necrosis. He was discharged without a definitive diagnosis, and three months later presented to an outside hospital with worsening symptoms. He was found to have progressively enhancing lesions in the brainstem for which comprehensive non-invasive testing was again repeated without clear diagnosis. He finally underwent brainstem biopsy which confirmed radiation necrosis.