RESUMO
Anisotropy functions for 169Yb sources used in interstitial brachytherapy are investigated. A comprehensive study of several factors affecting the angular dose distribution around four 169Yb seed models (Amersham International) has been undertaken. Absolute dose rates around 169Yb seed models 5, 8a, 8b and X1267 have been estimated by means of the EGS4 Monte Carlo Simulation System. An updated cross section library (DLC-136/PHOTX), binding corrections for Compton scattering and water molecular form factors were included in the calculations. Following the formalism developed by the Interstitial Brachytherapy Collaborative Working Group, anisotropy functions, F(r, theta), have been calculated and compared with other Monte Carlo results and whenever possible with experimental data. Excellent agreement is found with other Monte Carlo calculations. Considering the large experimental errors reported, a fairly good coincidence has been achieved between experimental and Monte Carlo data for models 8a and 8b. For model X1267 large discrepancies with experiment are obtained. Monte Carlo calculations for all seed models showed model 5 to be the least anisotropic and models 8b and X1267 to be almost identical. Statistical fluctuations can be drastically reduced computationally, offering an efficient alternative to measured data. Our results have estimated uncertainties of 0.5%-1.0% within one standard deviation everywhere excluding the longitudinal source axis, where uncertainties are below 3% up to 5 cm, this accuracy being excellent for clinical calculations.
Assuntos
Braquiterapia/instrumentação , Braquiterapia/métodos , Radioisótopos/uso terapêutico , Itérbio/uso terapêutico , Anisotropia , Método de Monte Carlo , Imagens de Fantasmas , Radiometria , Espalhamento de Radiação , ÁguaRESUMO
An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.