Measurement of the Dose in Small Tissue Volumes Surrounding "Point" Sources of Radioisotopes

Problems associated with the determination of the radiation dose received by a cell, or small tissue volume, from radioactive material inside the cell, or tissue itself, are given. Procedures are described which were used to determine the integral dose in a tiny mass of protoplasm. Since it was impossible to place an ionization chamber inside the cell, the cell and its surrounding medium were expanded to a much larger size by using a tissue- equivalent gas to represent the cell and medium. All linear dimensions were enlarged in the ratio of the density of tissue and that of the gas. Results, uncertainties, and principal sources of error are discussed. Schematic sketches are included of two wall-less ionization chambers which were developed for the measurements. The results for each isotope in absolute units are presented for spherical tissue volumes with radii up to about 200 mu in both integral and differential forms. Some of the biological applications of the results of the investigation are indicated. (C.H.)