With available information about thelocal radioresistance within the tumour volume the most general method formodelling the tumour response is to calculate the TCP at voxel level and thento integrate this function over the whole target volume (missing citation). Forpractical reasons studies so far proposed various hybrid models of dose painting by numbers and sub-volume boosting defining one ormore sub-volumes with different risk- or dose-levels. A general problem is thefact that the relevant tumour phenotypes discussed earlier can not be imaged atthe same time but only extended over several days in order to allow for aclearance of the different PET tracers between the scans. This complicates boththe acquisition of one final image to base the treatment planning on and thecalculation model itself. Apart from the usual setup uncertainties occurring inmultimodality imaging especially with individual images taken at differenttimes the spatial and temporal heterogeneities and changes of the biologicalsystem make the definition of a time-independent biological target volume (BTV(missing citation)) even more difficult. Nevertheless it is possible to definea biological target volume based on pre-treatment PET imaging.To take into account the uncertainties of the pre-treatment imaging thedynamics of the biological system during the treatment period and theradiation-induced biological changes of the tumour some groups proposed theconcept of adaptive radiotherapy based on additional functional imaging duringthe treatment period . While direct monitoring of the delivered dose duringtreatment is not yet possible monitoring the tumour response does indeed seemfeasible via repeated PET imaging . This kind of response assessment couldeventually allow for an individually adapted radiotherapy based on functionalimaging during the therapeutic process.In both pre-treatment and response-adaptive BTV definition a threshold valuecan help to quantify the relative changes in radiosensitivity within the tumourvolume as well as the effective radiosensitivity of the patient as such.Another crucial parameter is the time window used for evaluation. Recent publicationsagree in a time window of one to two weeks between the start of treatment andthe response assessment although further research has yet to be conducted .