Monte Carlo simulations of 241 Am α -particle energy distribution, LET distribution, and absorbed dose in a cell...

Monte Carlo simulations constitute a well-established methodology for the quantitative characterization of $\alpha$ -particle transport and energy deposition. Owing to their high linear energy transfer (LET) and limited range in tissue, $\alpha$ -particles are increasingly regarded as a promising modality for targeted cancer therapy.

The objective of this study was to perform a rigorous dosimetric characterization of an irradiation system utilizing a ²⁴¹Am surface source, with emphasis on quantifying the $\alpha$ -particle energy spectrum, their energy deposition characteristics, and the resulting absorbed dose in a cell monolayer model, given its relevance to radiobiological applications and its potential translational implications for targeted $\alpha$ -emitter radionuclide therapy (TAT).

A comprehensive geometric and material characterization of all irradiation system components was conducted. Cellular dimensions and culture medium thickness were quantified using confocal microscopy. A computational representation of the setup was implemented in the Monte Carlo N-Particle (MCNP) transport code to determine $\alpha$ -particle energy loss within relevant materials. Model predictions were validated against measurements obtained with a silicon semiconductor detector. Energy spectra of $\alpha$ -particles incident on the biological target were subsequently benchmarked against data acquired using CR-39 TASTRAK solid-state nuclear track detectors.

The simulations revealed homogeneous particle fluence across the exposed cellular monolayers. The simulated $\alpha$ -particle energies within the cellular volume extended up to approximately 1.6 MeV, with an associated LET distribution exhibiting a plateau in the range of 60-200 keV/ $\mu m$ and reaching peak values of about 250 keV/ $\mu m$ at the end of the particle range. Estimated absorbed dose within the cellular volumes indicates that, for the considered dimensions of the cell monolayer sample, a 1.01 $\mu m$ variation in culture medium thickness results in approximately a 36% difference in the calculated dose.

The present study emphasizes the necessity of precise dosimetric modeling for $\alpha$ -emitter irradiation platforms. We have calculated the dependence of energy deposition on the penetration depth of $\alpha$ particles with a specified energy distribution and LET. In the context of TAT, these results highlight the importance of selecting the optimal site for isotope injection, as well as accounting for changes in energy distribution caused by the traversal of alpha particles through tissue before reaching the tumor. Incorporating these empirically informed dose-geometry relationships into treatment planning may enhance the therapeutic precision and efficacy of targeted $\alpha$ -particle therapy.