Depth Dose Measurements for 100-, 120-, and 135-kv. Roentgen Rays

Abstract

The present trend toward larger x-ray doses in the treatment of superficial cancer makes the determination of the amount of radiation delivered to the underlying healthy tissue of greater importance. Such calculations, however, have been limited by the lack of complete depth dose tables for low and intermediate voltages, although numerous data are available for special technics or conditions (1, 5, 7, 10, 12). The present paper is a report on our phantom measurements for half-value layers from 1.0 to 8.0 mm. A1 (0.04 to 0.44 mm. Cu). Equipment and Measuring Instruments The apparatus was chosen with the aim of duplicating the conditions used clinically and consisted of a shockproof, oil-cooled x-ray tube energized by a half-wave, two-valve 140-kv. generator. The x-ray tube was mounted in a “rayproof” housing, and the total inherent filtration of the pyrex tube wall, oil, and bakelite window was equivalent to approximately 0.5 mm, aluminum. Comparative measurements were carried out, also, with an air-cooled shockproof tube having an inherent filtration of approximately 1 mm, aluminum. The tube peak kilovoltage was determined by means of a 12.5-cm. sphere gap which had previously been checked at constant potential against a Taylor high-voltage resistance unit. The milliamperage was maintained at a constant value by means of a stabilizer of the saturated core type. The radiation was measured with two bakelite thimble chambers coated on the inside with aquadag and having inside diameters of 0.52 cm. and 0.72 cm. respectively, with volumes of 0.40 c.c, and 1.27 c.c, Measurements at the surface and down to 5 cm. depth were made with both chambers, while at depths below 5 cm. the larger chamber only was used. A well shielded electrometer of the Wulf type was permanently connected to the chamber used. Choice of Phantom Material Preliminary experiments were made to determine the best phantom material for the ray qualities used. Water, being the main constituent of human tissue, has been preferred by several earlier investigators. It has, however, practical disadvantages. It is art electric conductor, which prevents its use with ionization chambers of the Failla extrapolation type (4) and, furthermore, since it is a liquid, it is difficult to obtain layers of definite thickness or cross section. Of the many solid substances used as phantom material, “masonite presdwood” has been the most satisfactory. “Masonite presdwood” is a wood fiber board manufactured by exploding wood chips by high pressure steam. Presdwood phantom measurements were first made by Quimby and collaborators (9), who found that masonite of unit density gave the same depth dose as water for 200-kv. radiation. In a previous paper (3) we reported that higher depth doses were obtained with masonite than with water when 40- to 50-kv. radiation was used, as applied in “contact” therapy. The absolute difference, however, was small, about 3 per cent at 1.0 em. depth, as the short target skin distance, 2.0 cm., caused a low depth dose for either material in accordance with the inverse square law.