Pearls & Oy-sters: Tumour-Like Mass Lesion Secondary to Primary CNS Vasculitis.

Primary CNS vasculitis (PCNSV) is uncommonly considered in the differential diagnosis of tumor-like lesions. This case report of tumefactive PCNSV highlights imaging features that should increase clinical suspicion for CNS vasculitis, potentially lending to earlier diagnosis and treatment. A 62-year-old man presented with a 1-month history of focal motor seizures and cortical sensory loss localizing to the right frontoparietal lobe. Noncontrast head CT was suggestive of glioma, resulting in intravenous dexamethasone administration and admission to neurosurgery. MRI appearance was atypical for glioma, with relative preservation of regional anatomy, intralesional microhemorrhage, and patchy peripheral enhancement. Despite normal CT angiogram, CSF, and serum inflammatory markers, brain biopsy was suggestive of lymphocytic vasculitis. Extensive workup for secondary causes was negative, and he was diagnosed with tumefactive PCNSV. Treatment with corticosteroids and cyclophosphamide resulted in sustained clinical and radiologic improvement. Tumefactive PCNSV is an angiogram-negative small-vessel vasculitis that has a lymphocytic histologic pattern. Tumefactive PCNSV constitutes over 10% of PCNSV cases and can be recognized by the presence of intralesional microhemorrhages, absence of diffusion restriction, and a patchy or nodular enhancement pattern. The most important mimicker is CNS lymphoma, which has a similar imaging and histologic pattern. If individuals with tumefactive PCNSV do not have a sustained immunotherapy response, repeat biopsy should be promptly performed.