Recently, there has been much interest in how to use information on patient-specific tumor biology and normal tissue function to individualize cancer treatment. In radiation therapy, dose may be escalated to radioresistant regions within a tumor, or regions of particular functional importance in normal organs may be preferentially spared. However, tumor and normal tissue biology may change during treatment, and adaptation of therapy may be necessary to ensure that optimal therapy is delivered. Furthermore, changes in tumor and normal tissue biology during early treatment may be predictive for the outcome of radiotherapy, and this information could be used for individual adaptation of the remaining part of the treatment. In the present study, we address variations that may occur in tumor and normal tissue radiobiological properties during radiotherapy, and how these may be related to the response to treatment. Moreover, we discuss the criteria for when to adapt treatment and how this adaptation should be performed. Finally, we discuss to what degree biologically adapted radiotherapy may be expected to improve treatment outcome and which issues need to be resolved for this strategy to reach its full potential.
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