The study reported in the present paper aimed to evaluate the effective energy (Eeff) 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 Eeff 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 Eeff by 8.8% and 14.6%, respectively. The validity of the calculated Eeff 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 Eeff 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.

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