Brain tumour. Light micrograph (LM) of a section of a primitive neuroectodermal of the brain. Primitive neuroectodermal tumour (PNET) is a malignant rare tumour, usually occurring in children and young adults under 25 years of age. The overall 5 year survival rate is about 53% It gets its name because the majority of the cells in the tumour are derived from neuroectoderm (forms brain tissue), but have not developed and differentiated in the way a normal neuron would, and so the cells appear "primitive". PNET belongs to the Ewing family of tumours. The mitotic index (number of dividing cells) is an important prognostic factor predicting both overall survival and response to chemotherapy in most types of cancer. The higher the mitotic count, the more likely the tumour is to have metastasized (spread). Magnification: x150 when printed at 10 centimetres wide. Human tissue.