Physical basis of dermatologic radiotherapy

Abstract

X-rays are part of the electromagnetic spectrum, which also includes gamma rays, ultraviolet light, visible light, infrared light, and radio waves. Electromagnetic waves are a form of energy; other forms are kinetic, thermal, nuclear, and gravitational energy. They may be described as the periodic variation of intensities of electric and magnetic fields at a given point (Selman 1960; Johns and Cunningham 1969). Electromagnetic waves travel in a straight line from their source and are independent of the presence of matter. They are characterized by the following parameters:

• 1. The frequency (v) is the number of waves passing a given point each second, or the number of vibrations per second.

• 2. The wavelength (λ) is the distance between two successive points in the wave that are characterized by the same phase of oscillation.

• 3. The velocity (c) is the distance the wave moves per second; it is a function of the frequency of the radiation and of the nature of the medium traversed. (In a vacuum or in air, all electromagnetic waves move at a speed of 299,793 km/sec, or 186,171 miles/sec.)