Effective energy assessment during breast cancer intraoperative radiotherapy by low-energy X-rays: A Monte Carlo...

The study reported in the present paper aimed to evaluate the effective energy (E_eff) of X-rays emitted from the surface of a bare X-ray probe and from different spherical applicators with various diameters, which are widely employed for low kV intraoperative radiotherapy (IORT) of breast cancer. A previously validated Monte Carlo model of the INTRABEAM system along with applicator diameters of 1.5-5 cm (with 0.5 cm increments) was employed for this purpose. The results show that the presence of the applicator can considerably harden the X-rays produced by the bare probe so that E_eff increases by a factor of about 2.6. Variations of applicator diameter also affects the X-ray effective energy. Specifically, increasing the applicator diameter from 1.5 to 3 cm and 3.5-5 cm resulted in an increase in the E_eff by 8.8% and 14.6%, respectively. The validity of the calculated E_eff values was confirmed by a reasonable agreement between the obtained probability density distributions (PDDs) for the full X-ray energy spectrum and those for the corresponding single effective energies, for different applicator diameters. The E_eff values obtained for different applicator diameters and the bare probe alone can be used as an alternative for the corresponding full energy spectra, in Monte Carlo-based dosimetry simulations of low-energy therapeutic X-rays, as well as for determining quality conversion factors of any ion chambers employed for low kV-IORT absolute dosimetry.