FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 μm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3–15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

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Applicability of pure propane gas for microdosimetric measurements in neutron fields was investigated using the FLUKA Monte Carlo code. Monoenergetic neutrons in the energy range 1 keV−20 MeV and the ISO-neutron sources such as 241Am-Be, 241Am-B, 252Cf and 252Cf + D2O were considered in the present study. The tissue-equivalent proportional counter (TEPC) simulated in the study was LET-1/2 (by Far West Technology) with site sizes 1, 2 and 8 μm. The study demonstrates that for a given site size, the TEPC filled with tissue-equivalent propane and pure propane gases produce similar microdosimetric distributions when the density of pure propane gas is lowered appropriately. For the ISO-neutron sources, the density of propane gas requires scaling by a factor 0.85. For the monoenergetic neutrons, depending upon the neutron energy, the values of scaling factors are in the range of 0.58–0.93.