An EGSnrc investigation of the air-kerma strength, dose rate constant, and radial dose function of 125I brachytherapy sources.

Titanium-encapsulated (125)I brachytherapy sources are in use for treatment of the eye, brain, and head and neck region, and for early stage prostate cancer. The photoelectric interaction of (125)I photons with titanium encapsulation generates Ti K X-rays (approximately 5 keV). According to the National Institute of Standards and Technology (NIST) 1999 air-kerma strength, S(k), standard, these X-rays should be excluded from S (k). We used the EGSnrc Monte Carlo code system to calculate the S(k) (including the contribution of approximately 5-keV X-rays), dose rate constant, and radial dose function for five different (125)I source models. Depending upon the source model, the contribution of 5-keV Ti X-rays to S(k) varies between 17.1 and 18.7%. Including these X-rays as part of S(k) would result in underestimation of the dose rate constant by up to 19%. The radial dose functions of the investigated sources are comparable to published studies that are based on an updated photon cross-section dataset.