Meningiomas are tumors that arise from the leptomeninges. They are generally benign tumors, and only rarely do they undergo malignant transformation. Meningiomas account for approximately 15% of all brain tumors. Meningiomas grow very slowly, often over many years. Because these tumors grow so slowly, they can sometimes become extremely large before being detected. This was especially true in past eras, before advanced imaging modalities were available (i.e. CT scans and MRI scans). Today, with our multitude of MRI and CT scan machines, meningiomas are most often detected when they are relatively small. MRI scanning is currently the best diagnostic tool for detecting and defining meningiomas.