Image-guided, adaptive radiotherapy of prostate cancer: toward new standards of radiotherapy practice.

The development and acceptance of new image-guided radiotherapy (IGRT) technologies have often been initiated with the treatment of prostate cancer. Imaging and tracking of the prostate during a treatment course has yielded a great deal of information about the motion and deformation of the gland during radiotherapy, and has led the way toward the development of more accurate treatment methods including dose-guided and adaptive strategies. Now, there is long-term experience with the use of fiducials and electromagnetic implantable beacons that give high-quality tracking of prostate motion. From analyzing these extensive tracking data sets, a clear understanding of prostate motion and its dosimetric significance has developed. This knowledge can now be used to define current expectations and guidelines for clinical care. The random nature of prostate motion requires daily localization if treatment is to be delivered with small margins. Interfraction motion can have a significant impact on prostate gland dosimetry, and even more of an impact on the seminal vesicles and possibly intraprostatic tumor areas. The dosimetric impact on normal structures (bladder/rectum) is less clear, and there are significant individual variations. Interfraction and intrafraction rotations and deformations of the prostate are routinely detected. The dosimetric impact of these motions of the prostate gland is minimal when daily localization is used, even when the treatment margins are small. However, deformations of the seminal vesicles, rectum and bladder are much more pronounced. The dosimetric impact of deformation of the rectum and bladder is highly variable among patients, and the clinical consequences remain unclear. Daily volumetric imaging and dosimetry may become quite important for these volumes. Due to the random nature of motion/deformation during prostate radiotherapy, adaptive radiotherapy ideally would be performed as an on-line process. On-line adaptive radiotherapy requires robust deformable registration and replanning programs. These are beginning to emerge in useful clinic applications.