Clinical possibilities with a new generation of radiation therapy equipment

The main steps in the development of isocentric megavoltage external beam radiation therapy machines are briefly reviewed identifying three principal types or generations of equipment to date. The new fourth generation of equipment presented here is characterized by considerably increased flexibility in dose delivery through the use of scanned elementary electron and photon beams of very high quality. Furthermore, the wide energy range and the possibility of using high resolution multileaf collimation with all beam modalities make it possible to simplify irradiation techniques, increase the accuracy in dose delivery and minimize the dose to normal tissues. The main design features are briefly described including scanning system, purging magnet, helium atmosphere and multileaf collimator. The new clinical possibilities with this fourth generation radiation therapy machines are briefly reviewed. Among the advantages are the ease of performing beam flattening, beam filtration and compensation, and the simplification of many treatment techniques using the wide spectrum of high quality multileaf collimated electron and photon beams. An example of treatment technique simplification is the application of the multileaf collimator for electron therapy where a dedicated electron applicator is generally not required owing to the small penumbra with the helium atmosphere in the treatment head. This has considerable advantages not least for arc therapy with electrons where the customary patient mounted tertiary collimators no longer are needed. Electron arc treatment of, for example, postoperative chest walls will therefore now be a fairly simple procedure. Finally, the possibility of doing conformation and more general isocentric treatments with non-uniform beams, by employing the multileaf collimator and the scanning system, is discussed. It is demonstrated that a few stationary non-uniform beams in many cases can generate almost as good dose distributions in the target volume as a fully optimized non-uniform conformation treatment. (author). 18 refs, 8 figs