MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (Co-60, Cs-137, Ir-192 and Yb-169) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 mu m. 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) over bar (F) and (y) over bar (D) values significantly. When the encapsulated Ir-192, Cs-137 and Co-60 sources are in water, (y) over bar (F) and (y) over bar (D) values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of (y) over bar, relative biological effectiveness (RBE) was obtained for the investigated sources. When Co-60, Cs-137 and Ir-192 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 Yb-169, RBE is about 2, independent of distance in water.