The application of high energy x rays and electron beams in radiotherapy

It has been more than 25 years since very high energy x rays generated by a betatron were first used in the treatment of cancer. The development of these machines along with linear accelerators quickly led to the use of high energy electrons as an additional modality in cancer management. In the intervening years, the physical and biological aspects of very high energy photon and electron beam therapy have been widely discussed in the literature, but documentation on the clinical efficacy of such modalities has been largely anecodotal. Thus the necessity for such costly equipment remains somewhat controversial. Radiotherapists are by no means in agreement as to what is the optimum energy for accelerator use in radiation therapy. This overview will review the physical characteristics of high energy x-ray beams and the factors effecting the dose distribution emphasizing both the advantages and the limitations of such beams. Specific examples of dose distributions appropriate for a given tumor distribution using both low energy x-ray beams (4 MV), high energy x-ray beams (25 MV), and x-ray beams in combination with electron beams of various energies will be presented.