Quantitative Studies of Relationships between Tumor Cell Ploidy and Dose Response to Ionizing Radiation in Vivo: Modification of Radiation Response in a Previously Irradiated Tumor

Abstract

A difference in dose-response to X-rays and Co60 gamma radiation was demonstrated in vivo between two closely related murine lymphocytic leukemias, one diploid and the other tetraploid. A diploid tumor was subjected to 1000 rads of X-irradiation at each of 50 sequential passages in vivo with 106 tumor cells (to minimize the chance of selection from small numbers of surviving cells). During the 50 irradiated passages, its radiation response was altered. A greater proportion of tumor cells survived any given radiation dose, increased nucleic acid content per cell was seen and tetraploid and aneuploid cells appeared in the population. The application of these findings to the "acquired radioresistance" reported by radiotherapists in the treatment of human cancer is discussed.