Evaluation of the new cesium-131 seed for use in low-energy x-ray brachytherapy

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope Cs-131. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U-1 in water, dose penetration characteristics similar to I-125 and Pd-103, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to I-125 and Pd-103 seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for Pd-103 and 60 days for I-125. The shorter half-life may offer significant advantages in biological effectiveness. (C) 2004 American Association of Physicists in Medicine.