Evidence for 'bystander effects' in vivo.

The observation of bystander effects in vitro have raised some serious questions as to the appropriate target size for calculation radiation dose. This has implications on the risk from ionizing radiation since dose is often directly related to radiation risk. This paper demonstrates that bystander effects do occur in vivo. It demonstrates that at low dose rates the bystander effects and risk are limited to the organ where the radiation dose is delivered. On the other hand, exposure to high radiation dose rates produces clastogenic factors that are released into the blood. These factors have been demonstrated both in vitro and in vivo and may influence risk in organs not directly exposed to the radiation. Bystander effects suggest that organs respond as a unit and are not just a bag of individual cells acting independently. Dose and risk must consider this unit.