AB022. Measurement of the ¹⁴N(n,p) reaction at neutron time-of-flight—European Organization for Nuclear Research: implications in dosimetry of boron neutron capture therapy

Background: In boron neutron capture therapy (BNCT), dosimetry calculations play a fundamental role when designing a treatment plan. The International Commission on Radiation Units and Measurements (ICRU) recommends that the delivered dose should have less than 5% deviation from the prescribed dose, in whatever form of radiotherapy, including BNCT. In dosimetry, one of the sources of uncertainty comes from nuclear data used in dose computations, but also from neutron transport simulations needed to compute the neutron fields in the irradiated region. The largest contribution to dose due to low-energy neutrons in human tissues is nitrogen, by means of the ¹⁴N(n,p) reaction. There were previous measurements of this reaction, but there are disagreements between data exceed 10%. For this reason, and in order to reduce the uncertainties in this cross-section and dose estimation in BNCT, the ¹⁴N(n,p) reaction cross-section was measured at the neutron time-of-flight (n_TOF) facility at European Organization for Nuclear Research (CERN).

Methods: The measurement used the time-of-flight technique and two detection set-ups: MicroMegas and DSSSD. The new cross-section data have been used to compute new estimations of the Kerma Factors for human tissues. A series of Monte Carlo simulations have been carried out with in order to test the implications of this high accuracy measurement, including effects in the neutron transport and the dose computations.

Results: The measurement allowed the determination of the cross-section in the range from 8 meV to 800 keV in a continuous measurement for the first time. The cross-section at the thermal point, crucial for BNCT, is 1.809±0.045 b. The uncertainty is below 2.5%, complying with ICRU recommendations. The value is in line but slightly lower (−1%) than the last Evaluated Nuclear Data File (ENDF) evaluation, with a larger difference (−6.6%) compared to the Japanese Evaluated Nuclear Data Library (JENDL) evaluated cross-section. The effect in the neutron transport, which could also affect other dose components as the neutron flux changes, is negligible at least to the needs for radiotherapy. The effect in dose, however, is small but noticeable, specially in low-energy neutron fields. In BNCT, 30–35% of the total dose in absence of boron is due to the ¹⁴N(n,p) reaction. Therefore, a variation of −1% in the dose estimates leads to a change around 0.30–0.35% below the estimates using previous data. In the presence of typical boron uptakes for BNCT treatments, the nitrogen-related dose contribution reduces to 16–20%, and thus, the change due to the new data falls around 0.16–0.20%.

Conclusions: The results from this new data confirm the reliability of the nuclear data, and the uncertainty in the dose estimates due to the cross-section of the ¹⁴N(n,p) reaction are reduced below the requirements for the practice of radiotherapy.

Keywords: Dosimetry; nuclear reactions; boron neutron capture therapy (BNCT)