Physical Basis for the High Skin Tolerance of Supervoltage Roentgen Rays

Abstract

The progressive increase of skin tolerance with the quality of the incident roentgen radiation has been appreciated for many years in the lower voltage range (1, 2). Recent clinical observations with two- and three-million volt roentgen rays (3) show further significant increases in skin tolerance; at these voltages, under proper conditions, only mild skin reactions are obtained even for high deep-tumor doses delivered through a single portal. It is the purpose of this paper to consider the physical basis for this high skin tolerance and to present an explanation which depends not only on the distribution of ionization produced by such radiation in the region immediately below the skin, but also on the energy of the individual ionizing particles. Nature of Biological Reaction to Radiation It is well understood that the biological effect of roentgen rays is due to the ionization produced by the absorption of radiation energy in tissue. The nature of this biological reaction is complex; it depends on the amount of ionization produced in the tissue and is affected by the rate at which this total dose is delivered. In the limit, a sufficient amount of ionization results in the immediate demise of the irradiated cell, presumably because of extensive change in the physiochemical structure of the cell protoplasm. At lower dose levels, death of cells may occur because of various kinds of direct cell and neighboring tissue injury; at the threshold dose the complete recovery of the cell is realized. These responses to irradiation vary extensively from one biological material to another and are dependent on the physical conditions of irradiation. The effective ionization in all cases is provided by energy removed from the tissue-traversing radiation by photoelectric absorption, Compton scattering, or pair-production. In these interactions of radiation with matter, electrons are accelerated with some or all of the energy of the incident radiation photons. These initial electrons in turn create, by collision with atoms along their path, the many secondary electrons which produce the principal biological effect. These secondary electrons, each possessing an energy generally well in excess of the binding energy of molecules, are capable of exciting and ionizing the atoms with which they in turn collide. The effect of such transfer of energy is often to break up the molecules containing the energized atoms. All manner of molecular disintegrations may occur, from a single break in a long chain molecule to fairly complete reduction to simple atoms. There is abundant evidence (4) that the ease of disruption, or radiosensi-tivity, is the greater the larger and more complex the molecular structure. Under some circumstances, the effect of energy transfer by electron bombardment may be the development of a more complex molecule, as in the formation of hydrogen peroxide by the irradiation of water.