Light microscope section through human tissue showing a meningioma. A meningioma is a tumour that forms on membranes that cover the brain and spinal cord just inside the skull. Specifically, the tumour forms on three layers of membranes that are called meninges. These tumours are slow-growing with as many as 90 percent being non-cancerous. Meningiomas are the most common type of tumours that originates in the central nervous system are occur most often in women that men.