ENTRANCE AND EXIT ABSORBED DOSE CHARACTERISTICS FOR A 6-MEV X-RAY GENERATOR.

Abstract

Because of the predominance of forward scatter when a high-energy photon beam interacts with matter, electronic equilibrium does not exist within the layer of material initially penetrated. In fact, passage through a minimum amount of substance is necessary before build-up to full value of the absorbed dose takes place. This minimum amount constituting the build-up layer is a function of both photon energy and atomic composition of the material with which the beam interacts. In addition, the degree of build-up, that is, the change in dose rate which takes place between the surface and the point at which peak dosage occurs, is also a function of the amount of material irradiated. Because of the clinical significance of skin reactions, accurate knowledge of the exit dose as a function of entrance portal area is also of interest. In the case of supervoltage x-rays, the exit skin dose may be significantly greater than the skin dose at the surface through which the beam is applied. This investigation was undertaken for the purpose of defining the relationship of surface, build-up, and exit dose values to that found under electronic equilibrium (peak dose) for an x-ray beam generated by bremsstrahlung production in a thick gold transmission-type target by 6-Mev monoenergetic electrons. The Failla (1) extrapolation chamber technic was designed to allow measurements to be made in conformance to the Bragg-Gray principle (2). With this chamber it is possible to extrapolate to a zero thickness entrance window. Such measurements for supervoltage x-rays have been reported by Baily and Beyer (3, 4). Since the chamber design allows a valid comparison of absorbed dose under conditions of both equilibrium (electronic) and nonequilibrium, measurements have been made of entrance, exit, and peak dose as a function of field size and also of center line depth-dose values for a 10 × 10-cm. entrance portal. These latter values are compared with those made in a water phantom with a cavity ionization chamber. Experimental The data were recorded with the bremsstrahlung produced by a linear electron accelerator. This generator yields a pulsed, monoenergetic, 6-Mev electron beam which strikes a thick transmission-type gold target, producing a typical bremsstrahlung spectrum. The resultant x-ray beam has a h.v.l. of 17.4 mm. Cu. The accelerator is pulsed between 250 and 450 times per second. Pulse length is 1.9 microseconds. It is nominally adjusted to deliver an average dose rate of 200 rads per minute in air atone meter as measured under electronic equilibrium for a 10 × 10-cm. field and operating at 300 pulses per second. This results in an instantaneous dose rate of 5,850 rads per second. Since this is a high delivery rate, recombination was investigated.