Comparative Isodose Charts for 200 Kv., 400 Kv., and 1,000 Kv. X-Rays

Abstract

From a physical point of view, the advantages of the higher voltages are twofold, namely, an increase in the percentage depth dose at the greater depths and a marked reduction in the amount of radiation scattered to the tissues immediately outside the primary beam. For voltages of the order of 200 kv. with large field sizes, the radiation scattered outside of the primary beam reaches a maximum, while with lower and higher voltages the lateral scattering is less. In fact, as pointed out below, with irradiation of a large field at 200 kv. the volume of tissue irradiated outside the beam may be as much as 50 per cent of the entire beam. This may be a serious handicap when considering the total amount of energy delivered to a patient. Another disadvantage of this large amount of scattering outside the beam at 200 kv. is discernible when using the multiple port technic. A close spacing of adjacent ports may result in the production of hot spots below the skin, causing damage to healthy tissue. Although there have been many publications of isodose curves for 200 kv., it was felt that, for purposes of comparison, measurements at this voltage should be repeated under the same conditions and in the same manner as the measurements at 400 and 1,000 kv. Measurements were therefore made at the three voltages with a thimble type ionization chamber (Victoreen condenser r meter), used with typical field sizes—5 × 5, 10 × 10, and 20 × 20 cm.—at a distance of 70 cm. A prestwood phantom was used as the absorbing and scattering medium. By measuring the intensity of the central ray at the surface, at various points of depth, and also at numerous points lateral to the central ray at the same levels, it was possible to express graphically the change in intensity at anyone depth when moving laterally from the central ray to the periphery of the beam. In this way data were obtained to make a complete isodose curve, as shown in Figures 1, 2, and 3. The actual spread of the beam was determined by exposing photographic films placed on the surface of the phantom and at a depth of 10 cm. The quality of the beam generated at 200 kv. was 155 X.U.; at 400 kv., 60 X.U.; and at 1,000 kv., 27 X.U. The 200 and 400-kv. beams were collimated by means of an aperture cut in 1/4-inch sheet lead which was suspended 7 cm. above the phantom. The 1,000-kv. beam was limited by an aperture in Linch lead, 30 cm. from the target, and a second aperture 4 3/4 cm. thick on the surface of the phantom. In order to facilitate comparison of the isodose curves for the three field sizes and the three voltages, only half of the isodose curves are shown, arranged in groups of three for each field size. If we consider the volume of tissue irradiated outside the beam to the 10 per cent line with respect to the total volume within the beam to the 20 cm. depth, we can estimate the change in volume when going from 200 to 1,000 kv.