Neutron capture-enhanced proton beam therapy outside the irradiation field: in vitro experiments

To evaluate the cell-killing effects of neutron capture-enhanced particle therapy (NCEPT) outside the proton beam irradiation (PBI) field.

Human osteosarcoma cells (MG63) were divided into control and NCEPT groups. Four hours before irradiation, the NCEPT group was exposed to ¹⁰B-boronophenylalanine. PBI was performed by targeting a gel bolus, with tissue culture plates placed outside the PBI field adjacent to the gel bolus. The bolus was irradiated with 12, 36, or 72 Gy. Cell survival fractions were calculated for wells adjacent to the bolus. Furthermore, the neutron fluence generated by PBI in the tissue culture plates was calculated using a Monte Carlo simulation. After 12, 36, and 72 Gy bolus irradiation, the cell survival rates in the control group were 103%, 95%, and 95%, respectively, whereas those in the NCEPT group were 84%, 75%, and 51%, respectively. The cell survival fraction in the NCEPT group was significantly lower than that in the control group (P < 0.01). The mean neutron fluence in the wells adjacent to the bolus was 1.24 × 10⁸ (1/cm²) at 72 Gy irradiation. Although NCEPT enhanced cell-killing outside the PBI field, the magnitude of this effect was modest.