X-RAY SPECTRAL MEASUREMENTS ON HIGH VOLTAGE AND SUPERVOLTAGE THERAPY BEAMS.

Abstract

The spectral distributions of radiation in actual treatment beams have been measured for 200 to 250 kv x rays, 2 Mv x rays, and cobalt-60 gamma rays. These measurements are part of a program to study the effects of wedge filters and collimating devices on therapy beams and to obtain more accurate information for the determination of roentgen-to-rad conversion factors. The measurements were made by using a scintillation counter to determine the radiation scattered by a thin sheet of low-atomic number material placed in the beam at the normal SSD. Measurement of the once-scattered spectrum, as suggested by Scrimger and Cormack (1), offers considerable advantages over direct measurement in the high-voltage region, since x-ray generators may be operated at normal tube currents, clinical beam collimators may be used, pin-hole alignment is no longer a problem, and scatter effects in the crystal collimator are greatly reduced. Spectra were measured by using a scintillation spectrometer to record the radiation at three scattering angles. Isotope sources were employed to determine the response function of the 2 in. NaI crystal; this function was written as a 32 × 32 matrix, which was inverted on a 7094 computer. A program was written so that data for each run, taken at 10 kev intervals, were multiplied by the matrix, given the appropriate Compton angle and scatter factor corrections, and printed out as the relative energy per kev interval. Spectral measurements have been made for all normal filter combinations at 200 kv, 220 kv, and 250 kv, with both open and closed end treatment cones. Some measurements have also been made on a 2 Mv x-ray beam and on the beam from a cobalt-60 teletherapy unit. Figure 1 shows the spectrum of the primary beam from a 2 Mev Van de Graaff generator having a tungsten target. This spectrum was determined by measuring the 90° scattered radiation. Measurements are being continued on beams of other qualities.