Abstract <p>The VITA accelerator-based neutron source has been proposed and developed at the Budker Institute of Nuclear Physics (Novosibirsk, Russia) for boron neutron capture therapy (BNCT), which includes a vacuum-insulated tandem accelerator for proton beam production, a lithium target for neutron generation through the ⁷Li(<i>p</i>, <i>n</i>)⁷Be reaction, and a neutron beam shaping assembly (BSA) with MgF₂ moderator for therapeutic beam generation. This paper presents a comparison of the results of numerical simulations of boron dose and γ-ray dose spatial distributions in a water phantom for three different neutron beam shaping assemblies. The investigations are carried out at the VITA accelerator-based neutron source in the Budker Institute of Nuclear Physics, Siberian branch, Russian Academy of Sciences, using a small neutron detector to measure these components of ionizing radiation. An agreement between the measured and simulated results is obtained. For the VITA accelerator neutron source that was manufactured and is being supplied to the Blokhin National Medical Research Center of Oncology in Moscow with the purpose of conducting clinical trials of the BNCT technique in Russia and the subsequent treatment of patients, the BSA is optimized by numerical simulation with consideration for the equipment, the presence of structural materials, and the wall separating the installation room from the irradiation room. The description of materials that make it possible to form a neutron beam that meets the requirements of the BNCT and the design of manufactured BSA is presented.</p>