Microdosimetry-based investigation of biological effectiveness of 252 Cf brachytherapy source: TOPAS Monte Carlo study.

OBJECTIVE: To investigate biological effectiveness of252 Cf brachytherapy source using Monte Carlo-calculated microdosimetric distributions.

APPROACH: 252 Cf 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 1 μm site size. Using these distributions, Relative Biological Effectiveness (RBE), mean Quality Factor (Q) and Oxygen Enhancement Ratio (OER) were calculated as a function of R.

MAIN RESULTS: The overall shape of the microdosimetric distributions are comparable at all the radial distances in water. However, slight variation in the bin-wise yield is observed with R. RBE, Q and OER are insensitive to R over the range 1-5 cm. Microdosimetric Kinetic Model (MKM)-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-based values are about 14.5 and 16, respectively. Theory of Dual Radiation Action (TDRA)-based RBE is 11.4. The calculated value of OER is 1.6.

SIGNIFICANCE: This study demonstrates the relative insensitivity of RBE, Q and OER radially away from the252 Cf source along the distances of 1 - 5 cm in water using microdosimetric techniques.