Skin Reactions and Tissue Heterogeneity in Electron Beam Therapy. Part I: Clinical Experience

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Abstract
The purpose of this paper is twofold: (a) to provide skin tolerance tables for electron beam therapy and (b) to evaluate the clinical significance of alterations in the electron beam depth dose produced by nonhomogeneous tissues. Although the electron beam was first used to treat patients almost fifteen years ago, a review of the literature has not provided tables which correlate the degree of skin reaction with dose, time, area, and anatomical location such as Paterson recorded for kilovoltage radiation in 1948 (10). In the published clinical papers, the alteration of dose distribution by the interposition of bone or air spaces in the path of the beam has not been emphasized as an important factor in treatment planning with electrons. Skin Reactions One of the first studies of the response of human skin to electrons was that of Haas et al. (5, 6) in a comparison of 17 Mev electrons versus 200 kv x rays. They established that a higher exposure dose of electrons was required to produce the same degree of erythema. From these doses of kilovoltage x rays and electrons, a relative biological effectiveness (RBE) of between 0.63 and 0.69 for electrons was calculated. This suggests that the skin-sparing effect of electrons is dependent upon a biologic difference between the two types of ionizing radiation and does not consider a build-up factor for high-energy electrons. Analysis of the collected case material from centers using the electron beam in Switzerland (13, 16), England (1, 14), and the United States (2, 7, 9, 15) offers a wide range of doses for the production of significant skin reactions. Heavy dry desquamations or moist reactions which healed in two to four weeks are described for a spectrum of doses from 3,400 rads in seventeen days to 9,500 rads in thirty-seven days with energy ranges from 8 to 40 Mev. Anatomical location of treatment field and field size have not in general been provided in the reports, and their possible influence on the degree of skin reaction has not been explored. Paterson observed that the tolerance of skin to kilovoltage radiation varies with the size of the area and the volume of tissue irradiated, the site, and the vascularity of the parts treated. The tolerance decreases as the area of irradiation increases. Correlating these factors with total dose and time, Paterson constructed tables of skin tolerance. In the treatment of the average patient with 250 kv radiation, a moist but rapidly healing reaction could be anticipated if the upper range tabular value was selected as the skin dose of radiation to be administered. The importance of anatomical location was also observed at the Curie Foundation where a decreasing order of skin tolerance was noted from the lateral face down to the chest wall.