Effects of split doses of X rays or neutrons on lung tumor formation in RFM mice

The effect of localized thoracic doses of X rays of fission neutrons delivered either as single doses or two equal doses separated by 24-hr or 30-day intervals on lung tumor formation in RFM mice was examined. Animals were killed 9 months after localized irradiation, their lungs were cleared, and the tumors were counted to determine both percentage incidence and tumors/mouse. After X-irradiation, recovery was observed during the 24-hr fractionation interval only when the total dose was on the dose-squared region of the dose-response curve obtained with single doses. At lower doses no effect (recovery of enhancement) was observed when the dose was split. Further, no significant increase in the split-dose effect was obtained by increasing the interval between doses from 24 hr to 30 days. After neutron irradiation, no effect of splitting the dose was observed at any dose, whether the interval between doses was 24 hr or 30 days. These data suggest that the primary mechanisms involved in the development of these tumors and in recovery are intracellular in nature and not influenced by intercellular interactions.