Modern radiotherapy (RT) techniques allow a highly precise and flexible deposition of the radiation dose in the tumor. As a consequence, high conformal tumor doses can be reached while sparing critical organs at risk. Hence, it seems to be very beneficial to increase the dose in the tumor according to functional and molecular information assessed by combined PET/CT imaging. Such functional image guided dose escalation is called dose painting. Two different dose painting approaches have been described to date: dose painting by contours (DPC) and dose painting by numbers (DPBN). DPC consists of delineating an additional functional target volume on the PET/CT images and prescribing this volume homogeneously to a higher dose, whereas DPBN integrates the functional image information directly into the treatment planning process in order to shape the dose in a locally varying manner according to the PET information. Before dose painting strategies can be applied routinely in clinical practice, a number of factors that potentially affect the accuracy and effectiveness of dose painting, such as image acquisition and reconstruction protocols as well as image quantification will have to be investigated in the future. Nevertheless, integration of functional PET/CT imaging seems to be the cornerstone for advancing RT towards personalized patient-based treatment approaches.
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