Boron neutron capture therapy (BNCT): Implications of neutron beam and boron compound characteristics

The potential efficacy of boron neutron capture therapy (BNCT) for malignant glioma is a significant function of epithermal‐neutron beam biophysical characteristics as well as boron compound biodistribution characteristics. Monte Carlo analyses were performed to evaluate the relative significance of these factors on theoretical tumor control using a standard model. The existing, well‐characterized epithermal‐neutron sources at the Brookhaven Medical Research Reactor (BMRR), the Petten High Flux Reactor (HFR), and the Finnish Research Reactor (FiR‐1) were compared. Results for a realistic accelerator design by the E. O. Lawrence Berkeley National Laboratory (LBL) are also compared. Also the characteristics of the compound p‐Boronophenylaline Fructose (BPA‐F) and a hypothetical next‐generation compound were used in a comparison of the BMRR and a hypothetical improved reactor. All components of dose induced by an external epithermal‐neutron beam fall off quite rapidly with depth in tissue. Delivery of dose to greater depths is limited by the healthy‐tissue tolerance and a reduction in the hydrogen‐recoil and incident gamma dose allow for longer irradiation and greater dose at a depth. Dose at depth can also be increased with a beam that has higher neutron energy (without too high a recoil dose) and a more forward peaked angular distribution. Of the existing facilities, the FiR‐1 beam has the better quality (lower hydrogen‐recoil and incident gamma dose) and a penetrating neutron spectrum and was found to deliver a higher value of Tumor Control Probability (TCP) than other existing beams at shallow depth. The greater forwardness and penetration of the HFR the FiR‐1 at greater depths. The hypothetical reactor and accelerator beams outperform at both shallow and greater depths. In all cases, the hypothetical compound provides a significant improvement in efficacy but it is shown that the full benefit of improved compound is not realized until the neutron beam is fully optimized.