These cells are shaped like a star, so when the tumors form they develop tentacles, which makes them difficult to remove surgically.
Additionally, the tumors advance rapidly. This is because astrocytes provide support to neurons and control the amount of blood that reaches them; so, when tumors form, they have access to a large number of blood vessels, helping cancerous cells to grow and spread very quickly.
Another reason that glioblastomas are so difficult to treat is their high rate of recurrence. This is partly due to a subpopulation of cells contained in the tumor called glioma stem cells (GSC) — a type of self-regenerating cancer stem cell that controls the growth of tumors.