Determination of Radiation Values in Lung Tissue with Variable Qualities of Radiation

Abstract

IN the use of high voltage roentgenray therapy, the question often arises in the minds of radiologists as to the depth dose of radiation given to lung tissue. What effect does the reduced density of lung tissue, with air cavities, have on the absorption and scattering of radiation? There have been no published data in the United States bearing on this subject, except a discussion of Duane's paper on “Measurement of Dosage by Means of Small Ionization Chambers,” by Glasser (1). In this discussion, Glasser gave data of ionization measurements made in water with the top of a small rectangular air cavity placed, first, near the surface in a water phantom, with the ionization chamber in the center of the field of radiation underneath the water phantom. The measurement of the intensity at this point was called 100 per cent. In the second position the top of the air cavity was moved one-fourth of the depth of the phantom from the surface of the water. With the ionization chamber in the same position, the measured intensity was 92 percent of the intensity in Position Number 1. In the third position, the top of the air cavity was placed one-half the depth of the water phantom from the surface. Ionization measurements of the intensity at this point showed it to be 86 per cent of the intensity when the air cavity was in Position Number 1. In the last and fourth position, the air cavity was placed on the bottom of the water phantom directly above the ionization chamber. Ionization measurements of the intensity at this point proved it to be 79 per cent of the intensity with the air cavity in Position Number 1. It is seen that by moving the air space from the surface of the water phantom to the bottom, there was a reduction in the quantity of radiation reaching the ionization chamber. There was a gradual reduction from the surface to the bottom of the water phantom, due to a change of the scattering conditions. The above experiment does not reproduce lung conditions: the lung is not moved about as is the air cavity. We wished to reproduce as nearly as possible conditions analogous to those in the instance of the treated patient. With this object we obtained a lung, so removed at autopsy as to enable us to connect it to an air system and maintain any desired inflation. The lung, connected to the air system, was immersed in a water phantom two centimeters below the surface of the water, the top of the lung always remaining two centimeters from the surface of the water, the expansion being mostly downwards and to some degree outwards. Ionization measurements were made with the lung in the following states of expansion: 1. Relative intensity measurements were made through 2 cm. of water and 3 cm. of lung tissue in a state of collapse at atmospheric pressure, obtaining measurements of the radiation at various depths in the water beneath the lung up to 20 centimeters.