Describe a rare presentation of a CNS collision tumor associated with a flow diverting stent.

A 49-year-old male with a history of subarachnoid hemorrhage from a ruptured right posterior cerebral artery aneurysm—secured with flow diverting stent three years prior—presented with progressive headaches, diplopia, and difficulty walking. Exam showed impaired right eye adduction and vertical movement, right hemifacial numbness and weakness, and left arm and leg weakness. MRI revealed a T2 hyperintense, heterogeneously enhancing, hyper-perfusing right mesial temporal lobe mass surrounding his stent, with leptomeningeal enhancement of the right oculomotor, trigeminal, and facial/vestibulocochlear nerves, and of the thoracic and lumbar spine. Extensive work-up was notable for positive serum Quantiferon, cerebrospinal fluid with elevated protein, markedly reduced glucose, and lymphocytic pleocytosis, and communicating hydrocephalus. Favored diagnoses after presentation at a multidisciplinary tumor board were CNS tuberculoma versus atypical reactive inflammation associated with his stent. Biopsy was deferred in favor of empiric steroid and anti-tubercular treatment with ventriculoperitoneal shunting. He had clinical and radiographic worsening on treatment, and ultimately underwent craniotomy for biopsy of the right temporal mass.

Pathology revealed a hypercellular neoplasm in a fibrillary background, with two distinct cell populations. Next generation sequencing revealed mutations in BRAF p.V600E and pTERT with homozygous deletion of CDKN2A/B in one population, and NF2 mutation with deletion of chromosome 22q in the other. Final diagnosis was a collision tumor containing WHO grade 3 pleomorphic xanthoastrocytoma (PXA) and WHO grade 1 meningioma.

Primary CNS collision tumors are exceedingly rare. The combination of PXA and meningioma has not been reported, and the pathogenesis is unclear. The colocation of this tumor and the prior subarachnoid hemorrhage raises the possibility that the hemorrhage arose from a small underlying PXA. After stenting, chronic foreign body reactive inflammation may have led to neoplastic meningeal proliferation and clinical worsening.