Background: Arachnoid granulation can sometimes show hypertrophy, developing extensively apart from the venous sinus, and in that case, a differential diagnosis should be made between this granulation and tumors. In this case, we hypothesized that cerebrospinal fluid was absorbed in the region of abnormal stains revealed by angiography.
Case description: A 67-year-old female with headache was admitted to our hospital. A plain radiograph revealed accumulated numerous osteolytic lesions in the right frontal bone. T1-weighted magnetic resonance (MR) images demonstrated mixed-intensity lesions. On the T2-weighted MR images, we observed that the lesions were mixed, with areas of the same intensities as gray matter and cerebrospinal fluid. An abnormal vascular stain from the frontal branch of the middle meningeal artery was confirmed. After a craniotomy, numerous white granular masses were observed. These masses had penetrated the dura mater and adhered rigidly to the arachnoid membrane. Histological examination revealed them to be normal arachnoid granulations and villi.
Conclusion: This case was diagnosed as an ectopic arachnoid granulomatosis. No case report has previously been published describing numerous arachnoid granulations away from the venous sinuses.