At present, target volume (tumour) delineation is characterised by the concepts of gross tumour volume (GTV), clinical target volume (CTV) and planning target volumes (PTV). Information from functional imaging may provide additional information for defining the ‘biological target volume’ (BTV)—a subvolume of the tumour that may indicate underlying radio-resistance. For example, hypoxic areas may be derived from PET- F-misonidazole (F-MISO) imaging and high tumour burden from 18-F-fluorodeoxyglucose positron emission tomography (FDG-PET)/CT or dynamic contract enhanced magnetic resonance imaging (DCE-MRI). A higher dose of radiation can then be delivered to the BTV (rather that the whole tumour) in a process known as dose painting (image adapted from Ling et al. )
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