Uniform homogeneous dosedistributions within the tumour volume include the risk of overdosing parts ofthe target volume while still failing to eradicate the high-risk sub-volumes(missing citation). Therefore a heterogeneous dose distribution taking into accountvariations within the tumor volume appears desirable. There are two strategiesto realise dose painting: sub-volumeboosting (missing citation) and dosepainting by numbers (missing citation).Sub-volume boosting applies anadditional dose to one or more smaller targets within the primary target. Thesub-volumes are defined by image segmentation on the basis of quantitativeinformation. In theory this is a binary technique using two discrete doselevels one for the primary target and a higher one for the sub-volumes.Dose painting by numbers is avoxel-based technique and aims to apply adapted dose levels based on amathematical transformation of the image intensity of individual pixels.Ideally an individual dose is applied to each voxel. This method provides ahigher degree of biological conformity but also places higher demands on theimaging and radiotherapy systems.Combinations of the two strategies define several sub-volumes each assigned toa different individual dose. The functional images required to obtaininformation about potential high-risk structures can be acquired usingcombinations of CT and PET scans. Various biomarkers each connected toparticular PET tracers have been identified to be related directly to thetherapeutic response and an increased risk of recurrence. The three mostcommonly considered biological characteristics are glycolytic metabolismproliferation and hypoxia . Although there are favorable tracers for each ofthese tissue properties it has to be noted that any tracer usually depends notsolely on one but on several factors.