Brain tumour. Coloured computed tomography (CT) scan of a high grade astrocytoma (green), a form of brain tumour. The white lines and purple cross are part of a system to precisely specify its co- ordinates for a biopsy (tissue-sampling operation). High grade astrocytoma is an aggressive cancer that arises from cells in supportive tissue in the brain. It grows steadily, infiltrating the surrounding brain; this may eventually cause increased pressure inside the skull. Treatment is usually surgery followed by radiotherapy.