## 1 Introduction  
Boron neutron capture therapy (BNCT) is a unique radiotherapeutic approach for treating cancer, relying on the \({}^{10}\)B(n, \(\alpha\))\({}^{7}\)Li nuclear reaction when stable \({}^{10}\)B interacts with thermal neutrons. The possibility of exploiting the neutron capture reaction to treat tumors was introduced by [PERSON] in 1936, laying the foundation for the development of boron neutron capture therapy (BNCT) [1]. The first application of BNCT took place in 1951, targeting a patient with malignant glioma. This groundbreaking endeavor utilized the existing nuclear research reactor at the Brookhaven Graphite Research Reactor [2]. A comprehensive overview of the state of BNCT based on reactor sources of neutrons was provided in a technical document published by the International Atomic Energy Agency in 2001 [3]. More recent advances in the field are summarized in a new IAEA report, published in 2023 [4]. BNCT is particularly promising for treating locally invasive malignant tumors due to the high neutron-capture cross-section of \({}^{10}\)B and the short ranges of its resulting alpha particles and recoiling \({}^{7}\)Li nuclei [1]. These high linear energy transfer (LET) particles, with LET values of approximately 150 keV\(\upmu\)m\({}^{-1}\) for alpha particles and 175 keV\(\upmu\)m\({}^{-1}\) for \({}^{7}\)Linuclei, deposit their energy within a small tissue volume, typically within micrometers. This selective energy deposition offers potential precision in cancer cell destruction while sparing surrounding healthy tissue. The alpha particles and \({}^{7}\)Li nuclei have approximate ranges of 4.5 \(\upmu\)m and 10 \(\upmu\)m [3; 4]. The key advantage of BNCT lies in the selective uptake of boron-loaded carriers by cancer cells, ensuring a higher concentration of boron atoms in cancerous tissue compared to healthy tissue. This selective uptake enables additional doses from alpha and lithium particles, effectively sterilizing cancer cells while limiting damage to healthy tissue [5]. BNCT presents several dose components, including the 'boron dose' (alpha and lithium-ion disintegration products), fast neutron dose, nitrogen capture proton dose, and gamma doses from the neutron source and from capture and scattering of neutrons in the beam line structures, as well as those produced in the body from hydrogen capture reactions. Accurate dosimetry in BNCT is essential due to its various radiation components. It is also essential to minimize radiation components other than those from thermal neutrons [3; 4]. Microdosimetry, involving the measurement of various components contributing to the total dose [6; 7], has emerged as a promising tool for characterizing radiation field quality in BNCT. The first measurements were conducted in 1992 [6] and subsequently discussed in a review paper [7]. Since then, both Monte Carlo simulation studies [8; 9] and experimental measurements using tissue-equivalent proportional counters (TEPCs) have explored microdosimetry in the context of reactor-based BNCT [10; 11; 12; 13; 14; 15; 16] and accelerator-based BNCT [17]. Given the diverse aspects of the investigated radiation fields (gamma component, fast neutron component, epithermal/thermal neutron field) and the variations in the employed detectors (different sensitive volume shape, simulated site size, and boron concentration, etc. ), the literature microdosimetric distributions are not directly comparable with those presented in this work. However, the measurements by [PERSON], [PERSON], and [PERSON] [6; 10; 11] were conducted at similar site sizes of 2, 0.5, and 1 \(\upmu\)m, respectively; therefore, the pure BNC component is expected to be similar to ours. No measurement has been reported for variable boron concentrations to date. The dose resulting from the boron neutron capture (BNC) reaction is directly proportional to the concentration of \({}^{10}\)B.