II. Dosage in Irradiated Soft Tissue and Bone

Abstract

Quantitative estimation of tumour dosage is made possible by substituting for the patient a so-called “phantom” of extremely simplified form and composition. The tumour dose is usually determined by means of measurements made in beams of radiation entering rectangular tanks of water, blocks of wax, pressed wood or other materials which approximate in absorption and scattering properties to the soft tissues of the body. The limitations of this approach to the dosage problem however were soon realised and, for example, Quimby and her colleagues (1934), and Wilson and Myers (1936), made measurements of dosage inside a cadaver which showed particularly the divergencies caused by the presence of bone and of air-filled lung tissue. Later, Mayneord (1937) made calculations of energy absorption in tissues and materials which differed from air in effective atomic number. Some attention has been given recently in this country to the effect on the dose distribution of the larger discontinuities of structure in the body and also to the study of dosage in bone itself (Spiers, 1946, 1949; Wilson, 1950; Lamerton, 1950). These two aspects of the problem are illustrated by the absorption data shown in Figs. 6 and 7 of the earlier paper (Spiers, 1946). The first of these figures shows the differences in the relative linear absorption of bone, muscle and fat which will largely determine the dose distribution in the body; the second gives the energy absorption per c.c. per r in these three kinds of tissue and indicates the way in which increased energy absorption per röntgen will occur in bone at the longer wavelengths.