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Planning volumes for a patient with WHO Grade 4 glioma (glioblastoma). (a) Planning CT showing contrast-enhancing tumour. (b) The GTV is the visible tumour. (c) A margin for microscopic spread has been added to make the CTV; the margin is the same in all directions except that it is restricted by the skull. (d) The PTV has been added outside the CTV to account for uncertainties in planning and execution of treatment; this extends beyond the inner table of the skull.
