Target volume definition in radiation therapy.

Target volume definition in radiation therapy is a broad field of interdisciplinary research. We give a brief history of clinical research in this field and outline some remarkable steps which led to the well-defined target volume concepts. The challenges in target volume definition for high-precision conformal radiation therapy are discussed, and possibilities of improving target volume definition, such as the integration of modern imaging modalities and the use of computer-based systems to support the radiation oncologist are indicated, as well as novel techniques for increasing the accuracy of patient positioning. All these tools should be evaluated with regard to their potential for increasing the therapeutic ratio and, as appropriate, should be implemented in clinical practice. However, target volume definition is a complex process influenced by many factors, currently under investigation. While questions remain in this field, and the impact of the influencing factors is not defined, the process of target volume definition should remain the subject of clinical research.