Intercomparative studies of brachytherapy techniques combined with 252CF Boron neutron capture therapy

Microdosimetry-based investigation of biological effectiveness of252Cf brachytherapy source: TOPAS Monte Carlo study

Objective.To investigate biological effectiveness of252Cf brachytherapy source using Monte Carlo-calculated microdosimetric distributions.Approach.252Cf source capsule was placed at the center of the spherical water phantom and phase-space data were scored as a function of radial distance in water (R= 1-5 cm) using TOPAS Monte Carlo code. The phase-space data were used to calculate microdosimetric distributions at 1μm site size. Using these distributions, Relative Biological Effectiveness (RBE), mean quality factor (Q̅) and Oxygen Enhancement Ratio (OER) were calculated as a function ofR.Main results.The overall shapes of the microdosimetric distributions are comparable at all the radial distances in water. However, slight variation in the bin-wise yield is observed withR. RBE,Q̅and OER are insensitive to R over the range 1-5 cm. Microdosimetric kinetic model based RBE values are about 2.3 and 2.8 for HSG tumour cells and V79 cells, respectively, whereas biological weighting function-based RBE is about 2.8. ICRP 60 and ICRU 40 recommendation-basedQ̅values are about 14.5 and 16, respectively. Theory of dual radiation action based RBE is 11.4. The calculated value of OER is 1.6.Significance.This study demonstrates the relative insensitivity of RBE,Q̅and OER radially away from the252Cf source along the distances of 1-5 cm in water.

Dosimetric effects of composition, location and size of tissue heterogeneities on 252Cf neutron brachytherapy

The purpose of this study is to evaluate the effect of tissue heterogeneities on dose distribution in Californium-252(²⁵²Cf) neutron brachytherapy. The effect of location and size of heterogeneity on dose distribution was also evaluated. Neutron and photon dose rate distributions were determined in a water phantom in presence of air, lung, soft tissue and bone heterogeneities using MCNPX code. To benchmark the Monte Carlo simulation of the ²⁵²Cf source, air kerma strength(S_KN), dose rate constant (Ʌ_N) and radial dose function (g_N(r)) were calculated and compared with previously reported data. Results showed a considerable reduction of neutron dose rate (up to 66%) inside heterogeneities, especially in air and bone heterogeneities, while the reduction of total photon dose rate was found less significant (up to 10%). In the presence of a heterogeneity, dose rate, fluence and energy spectrum were significantly different with respect to the homogenous phantom. The contribution of photon dose to the total dose in the presence of air and bone was dominant, compared to the neutron dose, whereas this photon contribution was reduced after passing the heterogeneity. As the bone heterogeneity size was increased from 1 × 1 × 1 cm³ to 1 × 3 × 1 cm³, the total dose and neutron energy fluence decreased of about 50% and 70%, respectively.