Reactive oxygen species formation and bystander effects in gradient irradiation on human breast cancer cells

Ionizing radiation (IR) in cancer radiotherapy can induce damage to neighboring cells via non-targeted effects by irradiated cells. These so-called bystander effects remain an area of interest as it may provide enhanced efficacy in killing carcinomas with minimal radiation. It is well known that reactive oxygen species (ROS) are ubiquitous among most biological activities. However, the role of ROS in bystander effects has not been thoroughly elucidated. We hypothesized that gradient irradiation (GI) has enhanced therapeutic effects via the ROS-mediated bystander pathways as compared to uniform irradiation (UI). We evaluated ROS generation, viability, and apoptosis in breast cancer cells (MCF-7) exposed to UI (5 Gy) or GI (8-2 Gy) in radiation fields at 2, 24 and 48 h after IR. We found that extracellular ROS release induced by GI was higher than that by UI at both 24 h (p < 0.001) and 48 h (p < 0.001). More apoptosis and less viability were observed in GI when compared to UI at either 24 h or 48 h after irradiation. The mean effective doses (ED) of GI were ~130% (24 h) and ~48% (48 h) higher than that of UI, respectively. Our results suggest that GI is superior to UI regarding redox mechanisms, ED, and toxic dosage to surrounding tissues.