Quantitative Relationships between Radiation Dose and the Reproductive Capacity of Tumor Cells in a Mammalian System in Vivo

Abstract

The ability to grow mammalian cells in clonal culture provided the first direct method for measuring the destruction of the reproductive capacity of the cell by ionizing radiation (1). The first system for similarly studying the reproductive capacity of tumor cells in the intact mammalian host was provided by the elegant work of Hewitt and Wilson (2–5). By dividing the number of tumor cells from unirradiated leukemic donor mice necessary to produce tumor “takes” in 50 per cent of recipient mice of the same strain, by the number of tumor cells from irradiated donor animals needed to produce similar 50 per cent “takes,” they obtained the “surviving fraction,” i.e., the percentage of cells remaining capable of unlimited reproduction after a given dose of radiation. The results reported by Hewitt and Wilson were in close agreement with those obtained in vitro and suggested that the reproductive integrity of many mammalian cells, regardless of species, and of normal or malignant origin, showed strikingly similar radiosensitivities. The methods used, however, provided considerable technical difficulty. The isolation of leukemic cells from the livers of donor animals and their accurate recognition (as compared to debris and normal tissue elements) and counting by phase-microscopy were tedious and required considerable skill and experience. At the National Cancer Institute, we have been most fortunate in having the ascites form of a lymphocytic leukemia in the DBA mouse strain, which lent itself admirably to studies of reproductive capacity and allowed much simpler manipulation. In this communication we shall outline system used and present the dose-response to x-rays thus obtained under oxygenated and anoxic conditions and with single and fractionated doses. Materials and Methods Mice: All mice used in this study were females of the DBA/2JN strain, supplied by the animal breeding service of the National Institutes of Health and were the products of brother-sister mating. They were three months old at the start of each experiment. Tumor: The P-388 lymphocytic leukemia was obtained from Dr. M. Potter, National Cancer Institute, at the 143d generation, having been carried by routine serial passage at weekly intervals in this laboratory over that period. The tumor produces copious, virtually bloodless ascites containing approximately 108 tumor cells per milliliter on the seventh day. The experiments reported in this communication encompass generations 245 to 270, but no change in either the character of the tumor or in mean time elapsed between inoculation and death has been noted at any time. The tumor suspension for routine passage was prepared as follows: The abdomen of a seven-day tumor-bearing mouse was cleansed with 70 per cent alcohol, and under sterile conditions 1–1.5 ml. of ascitic fluid was withdrawn by syringe and immediately diluted with an equal volume of ice-cold Tyrode's solution.