The measurement of radiation for medical purposes

The measurement of radiation for medical purposes is primarily concerned with the measurement of absorbed energy either at a given element of mass within the irradiated tissues or throughout the body as a whole. The ionization method of measuring X rays and gamma rays is briefly described in the paper and the fundamental nature of the röntgen is discussed. The distribution of radiation throughout a body subjected to X or gamma irradiation is next considered, and examples given of the variation of dosage-rate in typical cases along the axis of a beam sent into soft tissues. This leads to a study of "isodose surfaces" and some simple geometrical methods of obtaining them. The relationships between the dose in röntgens and the energy absorbed per unit mass of material or the energy flux per square centimetre of cross-section of the beam are indicated. The second problem of total energy absorbed is then attacked by integrating the product of the dose and element of mass throughout the whole mass of interest, thus obtaining the so-called "integral dose". Again, simple geometrical and analytical methods of deducing the values of the quantities involved are described. The significance of total energy absorption in many problems (as, for example, that of protection against stray radiation) are indicated. Finally, the total amounts of energy involved in the therapeutic use of ultra-violet and infra-red radiations are compared with those of X rays and gamma rays, and a plea is entered for the correlation of all these radiations on an energy basis.