There is growing concern over the effects of medical diagnostic procedures on cancer risk. Although numerous studies have demonstrated that low doses of ionizing radiation can have protective effects including reduced cancer risk and increasing lifespan, the hypothesis that any radiation exposure increases cancer risk still predominates. In this study, we investigated cancer development and longevity of cancer-prone Trp53 ^/– mice exposed at 7–8 weeks of age to a single 10 mGy dose from either a diagnostic CT scan or gamma radiation. Mice were monitored daily for adverse health conditions until they reached end point. Although the median lifespan of irradiated mice was extended compared to control animals, only CT scanned mice lived significantly longer than control mice (P < 0.004). There were no differences in the frequency of malignant cancers between the irradiated and control groups. Exposure to a single CT scan caused a significant increase in the latency of sarcoma and carcinoma (P < 0.05), accounting for the increased lifespan. This study demonstrates that low-dose exposure, specifically a single 10 mGy CT scan, can prolong lifespan by increasing cancer latency in cancer-prone Trp53 ^/– mice. The data from this investigation add to the large body of evidence, which shows that risk does not increase linearly with radiation dose in the low-dose range.