Mitchel, R. E. J., Jackson, J. S., Morrison, D. P. and Carlisle, S. M. Low Doses of Radiation Increase the Latency of Spontaneous Lymphomas and Spinal Osteosarcomas in Cancer-Prone, Radiation-Sensitive Trp53 Heterozygous Mice. Radiat. Res. 159, 320–327 (2003).Mice heterozygous for Trp53 are radiation-sensitive and cancer-prone, spontaneously developing a variety of cancer types. Osteosarcomas in the spine lead to paralysis, while lymphomas lead rapidly to death, distinct events that provide objective measures of latency. The effects of a single low-dose (10 or 100 mGy), low-dose-rate (0.5 mGy/min) 60Co γ irradiation on lymphoma or spinal osteosarcoma frequency and latency, defined as time of death or of onset of paralysis, respectively, were examined. Compared to spontaneous lymphomas or to spinal osteosarcomas leading to paralysis in unexposed mice, an exposure of 7–8-week-old Trp53 /− mice to 10 or 100 mGy had no significant effect on tumor frequency, indicating no effect on tumor initiation. All tumors are therefore assumed to be of spontaneous origin. However, a 10-mGy exposure reduced the risk of both lymphomas and spinal osteosarcomas by significantly increasing tumor latency, indicating that the main in vivo effect of a low-dose exposure is a reduction in the rate at which spontaneously initiated cells progress to malignancy. The effect of this adaptive response persisted for the entire life span of all the animals that developed these tumors. Exposure to 100 mGy delayed lymphoma latency longer than the 10-mGy exposure. However, the 100-mGy dose increased spinal osteosarcoma risk by decreasing overall latency compared to unexposed control mice. That result suggested that this higher dose was in a transition zone between reduced and increased risk, but that the dose at which the transition occurs varies with the tumor type.