A novel methodology to estimate pileup effects and induced error in microdosimetric spectra

Microdosimetry provides a superior characterization of the radiation field compared to conventional LET-based methodology, and for this reason it has become increasingly attractive for quality assurance in particle therapy. However, the typical particle rates used in the treatments lead to pileup, which distorts the experimental spectra, and thus compromises the accuracy of microdosimetric measurements, limiting their use in clinical settings. In this work, we investigated the pileup in a spherical Tissue Equivalent Proportional Counter (TEPC), and developed an algorithm to evaluate the contribution of this effect to the measured spectra. We exposed the TEPC to 11 and 70 MeV proton beams, and collected the microdosimetric spectra at rates in the $10^3$-$10^6$ pps range. Using a combination of GEANT4 Monte Carlo simulations and experimental data, we develop an algorithm capable of estimating the pileup probability affecting experimental measurements. The data suggest that the pileup probability has a linear increase with rate until it reaches a value of $15 \pm 3 \, \%$ at $28.2 \times 10^3$ pps, at which point it begins to saturate. Additionally, the outcomes from the comparison of the two proton energies suggest that the methodology used to estimate pileup can be used to predict the effects in clinical proton beams with similar energies. This same methodology can be applied to any type of microdosimetric measurement making it a reliable tool for addressing pileup issues and facilitating the application of microdosimetry in clinical settings.