DIAGNOSTIC NEURORADIOLOGY Tumor-mimicking primary angiitis of the central nervous system: initial and follow-up MR features Youkyung Lee & Ji-hoon Kim & Eunhee Kim & Sung-Hye Park & Yoo Jeong Yim & Chul-Ho Sohn & Kee-Hyun Chang Received: 17 April 2009 / Accepted: 2 June 2009 / Published online: 16 June 2009 # Springer-Verlag 2009 Abstract Introduction Primary angiitis of the central nervous system (PACNS) is an extremely rare vasculitis of unknown etiology. The purpose of this study was to describe the initial and follow-up magnetic resonance (MR) imaging features of the tumor-mimicking PACNS. Methods We retrospectively reviewed a total of 21 initial and follow-up brain MR images obtained in four patients with biopsy-proven PACNS mimicking brain tumor on MR images during the periods from 1 to 8.1 years. In the initial study, diffusion-weighted imaging (DWI; n =4), MR angio- gram (n =4), conventional catheter angiogram (n =3), perfusion MR (n =1), and computed tomography (n =1) and proton MR spectroscopy (MRS; n =2) were included. The lesions of the brain were qualitatively assessed in terms of location, number, size, shape, signal intensity, absence or presence of hemorrhage, enhancement pattern, and changes on the follow-up studies. Results Initially, the lesion manifested as single suprasellar (n =1) and frontal hemispheric (n =1) mass and as multiple- enhancing lesions in the unilateral supratentorial hemi- sphere (n =2). A patient showed steno-occlusive lesions in the internal carotid and middle cerebral arteries. DWI, perfusion imaging, and MRS revealed inconsistent findings among the patients. On the follow-up studies, a patient had two relapses but there was either significant decrease in size and extent or disappearance of the lesions with immuno- suppressive therapy in all patients. Conclusion Tumor-mimicking PACNS shows variable fea- tures on initial MR images but shows good responses to appropriate immunosuppressive therapy on follow-up MR images. Keywords Primary central nervous system vasculitis . MRI scan . Angiography Introduction Primary angiitis of the central nervous system (PACNS) is an extremely rare vasculitis of unknown etiology exclu- sively affecting the CNS without systemic disease. It is characterized by nonspecific granulomatous or nongranu- lomatous inflammation of the media and adventitia of the small cerebral and leptomeningeal arteries and veins. The clinical features are protean and neuroimaging findings are diverse, making the diagnosis difficult. Although highly presumptive diagnosis may be made on the basis of positive cerebral angiogram, leptomeningeal and/or cortical biopsy remains the gold standard to confirm the diagnosis. The Neuroradiology (2009) 51:651–659 DOI 10.1007/s00234-009-0546-3 Y. Lee : J.-h. Kim (*) : E. Kim : Y. J. Yim : C.-H. Sohn : K.-H. Chang Department of Radiology, Seoul National University Hospital, 101, Daehangno, Jongno-gu, Seoul 110-744, South Korea e-mail: jhnkim@radiol.snu.ac.kr S.-H. Park Department of Pathology, School of Medicine, Seoul National University, Seoul, South Korea K.-H. Chang Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, South Korea K.-H. Chang Neuroscience Research Institute, Seoul National University Medical Research Center, Seoul, South Korea