Monte Carlo and experimental dosimetry of an I125 brachytherapy seed

We have performed a comprehensive dosimetric characterization of the Oncura™ model 6711 ¹²⁵I seed using both experimental [LiF thermoluminscent dosimetry (TLD)] and theoretical (Monte Carlo photon transport) methods. In addition to determining the dosimetric parameters of the 6711, this report quantified: (1) the angular dependence of LiF TLD energy response functions for both point and volume detectors in water, poly(methylmethacrylate) , and solid water media; and (2) the contribution of underlying geometric uncertainties to the overall uncertainty of Monte Carlo derived dosimetric parameters according to the National Institute of Standards and Technology Report 1297 methodology. The theoretical value for the dose rate constant in water was 0.942 cGy U^-1 h^-1 ±1.76% [combined standard uncertainty (CSU) with coverage factor k=1] and the experimental value was 0.971 cGy U^-1 h^-1 ±6.1%. Agreement between experimental and theoretical radial dose function values was well within the k=1 CSU, while agreement between experimental and theoretical anisotropy function values was within the k=1 CSU only after incorporating the use of polar angle-dependent energy response functions. The angular dependence of the relative energy response was found to have a complex and significant dependence on measurement medium and internal geometry of the source.