Physical aspects of megavolt electron therapy.

Abstract

Directed streams of electrons with controllable energies in the 1-to 3-Mev range have been found uniquely suitable for the treatment of radiation-sensitive disease at the skin. This new approach to superficial therapy (1, 2) has been under investigation by members of the staff of the Lahey Clinic in Boston and the Massachusetts Institute of Technology during the four-year period commencing August 1951 (3). The present paper will report on the physical aspects of such megavolt electron therapy, including particularly the technics of electron irradiation of small and of extensive skin areas, the dose distribution below the skin, the reduction of accompanying whole-body x-irradiation, and the avoidance of possible accidental overexposure. The effective treatment of radiation-sensitive lesions of the skin presents a variety of often difficult physical requirements. In the simplest case, the lesion may involve a single localized region of moderate size and superficial depth. Control by ionizing energy requires the delivery of an adequate dose and, if the lesion is malignant, the inclusion of an ample margin of healthy peripheral tissue. Such lesions can usually be controlled by conventional low-voltage x-ray therapy or by radioactive plaque technics. Only too commonly these lesions are adjacent to or overlie tissue structure whose tolerance for radiation is small. Figure 1A shows a typical rapidly growing epidermoid tumor near the inner canthus of the eye. On the other hand, many superficial lesions are extensive and involve in varying degree all or nearly all of the skin. This is true of certain stages of mycosis fungoides, inflammatory carcinoma, psoriasis, atopic dermatitis, and generalized exfoliative dermatitis. Such cases, except for palliation of the most troublesome areas, are often outside the capabilities of conventional x-ray therapy and still further beyond the more limited scope of the external radioactive plaque. Figure 2A shows a representative patient with advanced mycosis fungoides involving most of the skin surface. For such cases, the use of large fields of high-energy electrons may offer a uniquely effective method of treatment. Analysis of the wide range of clinical manifestations of superficial disease has shown that most lesions can be effectively irradiated by electrons of constant but controllable energy directed at normal incidence into the skin surface. Such electron therapy offers the hitherto unattained possibility of both limiting the radiation dose to the actual depth of tissue requiring treatment and of reducing the reaction in the radiosensitive skin. Moreover, mono-energetic electron sources can be arranged to irradiate either localized areas or, substantially, the entire skin surface, with short exposure times and with accurate control of the dosage.