AbstractID: 7041 Title: On the anisotropy function and anisotropy constant of Theragenics Model 200 103Pd "light seed" Since the publication of AAPM TG 43 report, Theragenics Model 200 103Pd seed, which has been widely used in prostate seed implants, has undergone small changes in its internal geometry resulting from the vendor’s transition from reactor-produced 103 Pd (“heavy seeds”) to accelerator-produced 103 Pd(“light seeds”). Based on theoretical simulations and calculations, the dose rate constant and the radial dose function of the seeds are nearly independent of the small geometry changes. However, the anisotropy function of the seeds is more sensitive to the small geometric changes and needs to be re-evaluated. Furthermore, the method to compute the anisotropy constant is controversial and further investigation is warranted. The anisotropy function of the 103Pd "light seeds" were measured in Solid Water phantom using micro LiF TLD chips at radial distances of 1 to 6 cm and at angles from 0 to 90 degrees with respect to the longitudinal axis of the seeds. In order to investigate the best choice for the anisotropy constant, a biological model, based on the continuous low dose rate irradiation linear-quadratic model, was used on prostate implants of 30 actual patients. It was found that at small radial distance the anisotropy function of the 103Pd "light seed" is lower than that of the 103Pd "heavy seed" reported in TG 43, and is in excellent agreement with the results of a Monte Carlo simulation reported by Weaver, except for points along the longitudinal axis. The anisotropy constant, calculated with the biological model, results in a value of 0.865.