DEPTH-DOSE TABLES FOR 6 MV X RAYS FOR USE WITH A CONSTANT SOURCE-LESION DISTANCE.

Abstract

IN view of the desirable depth dose characteristics of high-energy radiations for treating deep-seated tumors, many supervoltage radiation therapy machines of different designs, constructions, and energies have appeared in the market. One of the most recent additions is the Varian Associates 6 Mev linear electron accelerator for x-ray and electron therapy. This accelerator is designed with a can-tilevered treatment couch so that rotational-, multiple-, or single-port treatments can be administered without moving the patient. Of course, in such a system the source-skin distance will change with changes in the direction of the beam. To our knowledge, constant SLD dose tables for 6 Mv x rays are not available. In fact, relatively little depth-dose information has been published, and this has been for only one source-skin distance. We have prepared a set of tables2 of central axis depth doses for use with 6 Mv x rays at a constant source-lesion distance of 100 cm. Published data for 4 and 8 Mv x rays were used as a starting point in preparing the tables (1). Interpolations were made from the 4 and 8 Mv data to obtain a set of depth-dose values for 6 Mv for 100 em source-skin distance. These values compared favorably with our measurements and with reported data for 6 Mv x rays obtained by Haimson and Karzmark, by Chamberlain and Baily, and by Morgan (2–4). From these data other SSD values were obtained by assuming an inverse-square variation of intensity with distance. Calculated data for other SSDs also were in close agreement with our measured values. Methods and Results The radiation source was a 6 Mev linear electron accelerator developed by Varian Associates for supervoltage x-ray therapy. This accelerator utilizes a 1.5 mm thick fixed transmission-type gold target to generate the bremsstrahlung x rays. Factors are a stainless-steel beam-flattening filter of 12.3 mm axial thickness, a 1.5 mrn lead electron filter, a thin nickel vacuum chamber window, a glass mirror, and an ionization chamber in the beam in addition to the gold target, which is bombarded by the 6 Mev electrons at an oblique angle. We estimate that the inherent filtration in this unit is equivalent to about 1.3 em of lead. The emergent x-ray beam has an effective photon energy of 1.9 Mev with a h.v.l, of 18.0 mm Cu or 12.5 mm Pb. Our copper value agrees reasonably well with the 17.4 mrn reported by Chamberlain and Baily (3). The experimental values were based on measurements taken with a Victoreen thimble ionization chamber and roentgen ratemeter in a 12 X 18 X 24-in. water phantom. The front wall of the phantom tank was a 1 em thick sheet of Lucite of density 1.17 g/cm2. With the front surface of this phantom positioned at the treatment distance, maximum build-up occurred at a position equivalent to 1.5 g/cm2 in water.