Study of Back-Scattered Radiation and Depth Dosage Using a Transplantable Animal Tumor as an Indicator

Abstract

WHEN a beam of radiation strikes matter, some of the rays are scattered in all directions. Among other factors, the amount of scatter depends upon the quality of the radiation, the nature of the material upon which it impinges, the volume of the material, and the area of the beam. The relative amount of scattered radiation is different in different directions, being greatest in the forward and least in the backward direction (1). Many investigators have measured the scattered radiation by means of ionization chambers, photographic films, and various chemical and biologic media, placed upon the surface of or in water, paraffin, or the human body. The use of different materials combined with variations in experimental conditions has led to varying results and, therefore, much controversy. The work here reported was planned to determine the back-scattered radiation and depth dosage by means of tumor tissue at different depths in a block of paraffin. It is possible that the results of this study may throw some light upon the existing differences among physical, chemical, and biologic measurements of radiation. Study of Back-Scattered Radiation In a previous study (2) on the reaction of transplantable Mouse Sarcoma 180 to radiation of different wave lengths, we found that the radiosensitivity of this tumor to filtered roentgen rays was constant from time to time and that the change in the viability of the transplants was definite and easily measurable. Furthermore, the reaction was not influenced by slight changes in laboratory conditions, or other extraneous causes. In addition, the quantity of radiation required to produce the measurable changes was in the neighborhood of the amounts ordinarily used in therapy. Accordingly, in the present experiments a number of observations were made with Mouse Sarcoma 180 irradiated in air to determine whether or not the radiosensitivity of the tumor had remained the same as had previously been determined. In order to avoid uncertainties due to secondary radiations and the loss of tissue water, a special cellophane chamber was constructed. One layer of very thin cellophane (0.025 mm.) was stretched over an ordinary wooden embroidery hoop, with a diameter of 9.5 centimeters. On this was placed a piece of filter paper (0.13 mm.), slightly moistened with a Locke-Ringer solution at PH 7.0. Fragments of fresh tumor tissue, each weighing about 6 mg., and measuring 1.5–2.0 mm. in thickness, were then arranged in rows on the paper. A second embroidery hoop with a diameter of 12 cm., with one sheet of cellophane stretched across, was then placed over the first, to reduce evaporation and prevent bacterial contamination. Aseptic precautions were taken throughout. In the present experiments, the tumor fragments were subjected to radiation from a standard water-cooled Coolidge roentgen tube under the following conditions: 200 kv., 32 ma., 0.5 mm. copper and 1.6 mm. aluminum filter, 50 cm. distance and 400 sq. cm. field.