A Technique for Large-Field, Superficial Electron Therapy

The application of electron beams to radiation therapy has received increased attention in recent years. This interest stems largely from the special characteristics of the electron depth-dose distribution; the fall-off in depth dose near the end of the range of the electrons is particularly rapid for electron beams having energies less than about 10 Mev. Such an electron beam is suitable for the treatment of widespread, superficial lesions, where it is desirable to minimize the dose to deep tissues. All the common sources of super-voltage x-rays, apart from radioactive substances, are inherently capable of producing an external beam of electrons, though with varying degrees of complexity. Electrons of about the energy suitable for superficial therapy (several Mev) may be used to produce an x-ray beam which is sufficiently penetrating for the treatment of even the deepest lesions. The Stanford medical linear accelerator was designed to provide both x-rays and electron beams. The accelerator and its application to x-ray therapy have been previously described (1–4). The present report considers large-field electron therapy applications. With the techniques and apparatus to be described, it is possible to change from x-ray operation to electron operation, or vice versa, in approximately five minutes. The measurement of dose distributions confined to a depth of about 1.0 cm. requires special dosimetric techniques, which are also described. Treatment Technique The physical arrangement for electron therapy must be consistent with the following considerations: 1. Patients with superficial lesions frequently require irradiation over large areas of the body surface; in extreme cases the total area of the skin may require treatment. Acceptable uniformity of dose over the treatment area must be maintained. 2. The electron flux which reaches the treatment surface will be accompanied by a supervoltage x-ray background. Since in many situations this will constitute total-body irradiation, its contribution must be kept low. 3. In general, it is necessary to use the same machine for both x-ray and electron work. Consequently, it must be possible to make the changeover from one type of beam to the other rapidly and conveniently. 4. The technique should not impose severe demands on the size of the therapy room or its arrangement.