C Clifton Ling, Ph.D., Medical Physics Department, Memorial Sloan Kettering Cancer Center

IMRT has substantially reduces the volume of critical organs irradiated to high doses, and has permitted the increase of tumor dose without concomitant increase in normal tissue complication.  At MSKCC, a clinical trial in cancer of the prostate has accrued nearly two thousand patients and the prescription dose has been escalated to 81 Gy with 3D-CRT, and to 86.4 Gy using IMRT, with improved clinical results.  Of importance is the “dose-painting” capability of IMRT, i.e. to deposit radiation dose non-uniformly with the target and critical tissues to maximize the therapeutic ratio. 

The "new" concept of IGRT (image-guided radiotherapy) may appear "retrograde" since from the very beginning radiotherapy has been image-based.   However, the IGRT of the twenty-first century has its impetus in the concomitant but independent development of advanced imaging and radiotherapy methods, and indeed in the re-integration of imaging and treatment devices.  Specifically, stereotaxic or volumetric imaging devices are now combined with radiation treatment units to provide image-guidance in real-time (or almost real-time).  Perhaps the present approach would be more aptly called IG(RT)2, for radiotherapy with image-guidance in real-time.

Concomitant with the advances in radiation treatment, much development occurred in imaging, especially in CT, MRI and PET.  Totally unanticipated until the last decade is the impact of the human genome project and the emerging possibility of imaging biological processes.  Indeed, the spectacular advances in our understanding of cellular and molecular processes and their application to imaging are providing new types of images, with a wide spectrum of “biological” information, including metabolic, biochemical, physiological, functional and molecular. 

The improvement in the dose distribution conformality of IMRT, and the emerging ability of imaging that can give information about factors (e.g. tumor hypoxia, tumor burden) that influence radiosensitivity and treatment outcome, avail tremendous opportunities and bring new challenges.  Aspects of this ongoing synthesis of advances will be discussed, in what some would say is “physics meets biology”.