Theoretical and empirical investigations of KCl : Eu2+ for nearly water-equivalent radiotherapy dosimetry.

PURPOSE

The low effective atomic number, reusability, and other computed radiography-related advantages make europium doped potassium chloride (KCl : Eu2+) a promising dosimetry material. The purpose of this study is to model KCl : Eu2+ point dosimeters with a Monte Carlo (MC) method and, using this model, to investigate the dose responses of two-dimensional (2D) KCl : Eu2+ storage phosphor films (SPFs).

METHODS

KCl : Eu2+ point dosimeters were irradiated using a 6 MV beam at four depths (5-20 cm) for each of five square field sizes (5 x 5-25 x 25 cm2). The dose measured by KCl : Eu2+ was compared to that measured by an ionization chamber to obtain the magnitude of energy dependent dose measurement artifact. The measurements were simulated using DOSXYZnrc with phase space files generated by BEAMnrcMP. Simulations were also performed for KCl : Eu2+ films with thicknesses ranging from 1 microm to 1 mm. The work function of the prototype KCl : Eu2+ material was determined by comparing the sensitivity of a 150 microm thick KCl : Eu2+ film to a commercial BaFBr0.85 I0.15 : Eu(2+)-based SPF with a known work function. The work function was then used to estimate the sensitivity of a 1 microm thick KCl : Eu2+ film.

RESULTS

The simulated dose responses of prototype KCl : Eu2+ point dosimeters agree well with measurement data acquired by irradiating the dosimeters in the 6 MV beam with varying field size and depth. Furthermore, simulations with films demonstrate that an ultrathin KCl : Eu2+ film with thickness of the order of 1 microm would have nearly water-equivalent dose response. The simulation results can be understood using classic cavity theories. Finally, preliminary experiments and theoretical calculations show that ultrathin KCl : Eu2+ film could provide excellent signal in a 1 cGy dose-to-water irradiation.

CONCLUSIONS

In conclusion, the authors demonstrate that KCl : Eu(2+)-based dosimeters can be accurately modeled by a MC method and that 2D KCl : Eu2+ films of the order of 1 microm thick would have minimal energy dependence. The data support the future research and development of a KCl : Eu2+ storage phosphor-based system for quantitative, high-resolution multidimensional radiation therapy dosimetry.