  Background and purpose: Accelerator-Based Boron Neutron Capture Therapy is a
radiotherapy based on compact accelerator neutron sources requiring an
epithermal neutron field for tumour irradiations. Neutrons of 10 keV are
considered as the maximum optimised energy to treat deep-seated tumours. We
investigated, by means of Monte Carlo simulations, the epithermal range from 10
eV to 10 keV in order to optimise the maximum epithermal neutron energy as a
function of the tumour depth.
  Methods: A Snyder head phantom was simulated and mono-energetic neutrons with
4 different incident energies were used: 10 eV, 100 eV, 1 keV and 10 keV.
$^{10}$B capture rates and absorbed dose composition on every tissue were
calculated to describe and compare the effects of lowering the maximum
epithermal energy. The Therapeutic Gain (TG) was estimated considering the
whole brain volume.
  Results: For tumours seated at 4 cm depth, 10 eV, 100 eV and 1 keV neutrons
provided respectively 54 $\%$, 36 $\%$ and 18 $\%$ increase on the TG compared
to 10 keV neutrons. Neutrons with energies between 10 eV and 1 keV provided
higher TG than 10 keV neutrons for tumours seated up to 6.4 cm depth inside the
head. The size of the tumour does not change these results.
  Conclusions: Using lower epithermal energy neutrons for AB-BNCT tumour
irradiation could improve treatment efficacy, delivering more therapeutic dose
while reducing the dose in healthy tissues. This could lead to new Beam Shape
Assembly designs in order to optimise the BNCT irradiation.
