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-h 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, recovey was observed during the 24-h 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 or 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 h to 30 days. After neutron irradiation, no efect of splitting the dose was observed at any dose, whether the interval between doses was 24 h 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.