Towards multidimensional radiotherapy (MD-CRT): Biological imaging and biological conformality

Purpose: The goals of this study were to survey and summarize the advances in imaging that have potential applications in radiation oncology, and to e xplore the concept of integrating physical and biological conformality in m ultidimensional conformal radiotherapy (MD-CRT). Methods and Materials: The advances in three dimensional conformal radiothe rapy (3D-CRT) have greatly improved the physical conformality of treatment planning and delivery. The development of intensity-modulated radiotherapy (IMRT) has provided the "dose painting" or "dose sculpting" ability to furt her customize the delivered dose distribution. The improved capabilities of nuclear magnetic resonance imaging and spectroscopy, and of positron emiss ion tomography, are beginning to provide physiological and functional infor mation about the tumor and its surroundings. In addition, molecular imaging promises to reveal tumor biology at the genotype and phenotype level. Thes e developments converge to provide significant opportunities for enhancing the success of radiotherapy, Results: The ability of IMRT to deliver nonuniform dose patterns by design brings to fore the question of how to "dose paint" and "dose sculpt", leadi ng to the suggestion that "biological" images may be of assistance, In cont rast to the conventional radiological images that primarily provide anatomi cal information, biological images reveal metabolic, functional, physiologi cal, genotypic, and phenotypic data. Important for radiotherapy, the new an d noninvasive imaging methods may yield three-dimensional radiobiological i nformation. Studies are urgently needed to identify genotypes and phenotype s that affect radiosensitivity, and to devise methods to image them noninva sively, Incremental to the concept of gross, clinical, and planning target volumes (GTV, CTV, and PTV), we propose the concept of "biological target v olume" (BTV) and hypothesize that BTV can be derived from biological images and that their use may incrementally improve target delineation and dose d elivery. We emphasize, however, that much basic research and clinical studi es are needed before this potential can be realized. Conclusions: Whereas IMRT may have initiated the beginning of the end relat ive to physical conformality in radiotherapy, biological imaging may launch the beginning of a new era of biological conformality. In combination, the se approaches constitute MD-CRT that may further improve the efficacy of ca ncer radiotherapy in the new millennium. (C) 2000 Elsevier Science Inc.