Effect of host age on the transplantation, growth, and radiation response of EMT6 tumors.

The characteristics of EMT6 tumors in young adult (3- to 4-month-old) and aged (20- to 28-month-old) BALB/c KaRw mice were compared. The number of tumor cells implanted s.c. necessary to cause tumors in 50% of the injection sites was lower in aging than in young adult mice (69 and 138 cells for young adult mice versus 8.8 and 16 cells for aging mice in two experiments). The latent period of intradermally implanted tumors was shorter in aging mice than in young animals; however, the growth curves of established tumors were similar. The number and appearance of lung colonies after injection of cells i.v. and the pattern of spontaneous metastases were similar in young and aged animals. The cloning efficiencies (viabilities) of cells suspended from tumors in young adult and aging animals were the same (approximately 30% in both groups). Radiation dose-response curves for the cells of tumors in young and aging mice were different and suggested that the proportion of hypoxic cells was higher in tumors on aging animals. These findings suggest that both immunological and nonimmunological tumor-host interactions differ in young and aged animals and that such factors may influence the natural history of the tumor and the response of the tumor to treatment.