29 A new era in radiotherapy

The search for new radiation treatment techniques to improve local tumor control continues to represent a major challenge in the management of localized human cancer. Before CT scans became available for planning radiation treatments, tumor target volumes and the boundaries between tumor and normal organs were poorly defined. To reduce the risk of local relapse, radiation target volumes classically included a wide “safety” margin of surrounding normal tissue. Restrictions imposed by normal tissue tolerance often limited the radiation doses administered to less than optimal levels. CT-assisted treatment planning improved the ability to define tumor target volumes for radiotherapy. However, the limitations imposed by computer technology have confined the use of CT information in conventional 2D treatment planning to one or at most a few CT slices at or near the central axis of the radiation beam, whereas treatment planning at levels beyond the central axis is not based on detailed CT anatomical information. The introduction of three-dimensional conformai radiation therapy (3D-CRT) has heralded a new era in radiotherapy. Sophisticated computer-aided techniques are used to plan and deliver prescribed radiation doses, conforming the desired dose distribution to the entire 3D configuration of the tumor. The use of 3D imaging technology for treat -ment planning has reduced the risk of underdosing parts of the tumor. In addition, the effective exclusion of normal tissues from the volume carried to high radiation dose levels has allowed an escalation in tumor dose to levels beyond those feasible with conventional 2D radiotherapy. The aim is to improve local tumor control, as failure to control the primary tumors is associated with increased rates of both metastatic disease and mortality. Because of the increasing complexity of 3D treatment planning and the need to evaluate large numbers of competing plans, a critical development for realizing the full potential of 3D-CRT is the introduction of computer-aided optimization of treatment planning. In addition, the extraordinary increase in the complexity of treatment delivery requires the introduction of immobilization devices, computer-controlled treatment delivery machines with automated multileaf collimatore for beam shaping, on-line verification systems with a potential for feedback control loops to automatically correct set-up errors during treatment, and patient safety devices. Dosimetrie methods are being developed to compensate for the inaccuracy in 3D treatment caused by organ motion. Many of these components have been implemented at the Memorial Sloan-Kettering Cancer Center.