Microdosimetric

Microdosimetric, or microdosimetry, refers to the study of how ionizing radiation deposits energy within microscopic sites that are similar in size to cellular components. It focuses on energy deposition at the micrometer and sub-micrometer scales, where stochastic fluctuations influence biological outcomes. The central quantity is lineal energy y = ε/l, the deposited energy ε divided by the mean chord length l of the site, with units keV per micrometer. By collecting the distribution of y values across many independent sites, microdosimetry links physical energy deposition to cellular response.

Experimental approaches include tissue-equivalent proportional counters (TEPCs) that mimic energy deposition in tissue-like volumes, producing lineal energy spectra f(y). Complementary computational methods use track-structure and Monte Carlo simulations to model energy deposition in predefined site geometries, including nanodosimetry for nanometer scales. Derived microdosimetric quantities such as the dose-mean lineal energy and the frequency-mean lineal energy provide summaries that correlate with biological effects, and they can be related to relative biological effectiveness for different radiation qualities.

Applications include optimization of radiotherapy with heavy charged particles, assessment of risks from high-LET radiation, and evaluation of space radiation exposures. The microdosimetric approach helps explain why two radiations with equal macroscopic dose may produce different biological effects due to differences in the shapes of their y distributions. It also informs radiobiological models that connect energy deposition to cell killing, mutation, or other endpoints.

Challenges in microdosimetry include accounting for complex tissue geometry, chemical stage effects, and variability in cellular sensitivity. Limitations of microdosimetric measurements and spectrum interpretation are mitigated by combining experimental data with detailed simulations and by extending to nanodosimetry to capture DNA-scale interactions.