Cesium-131 seeds for permanent implants.

Abstract

PERMANENT interstitial implants with radon-222 and gold-198 seeds are of special value in cancer therapy because they are easier to carry out, cause less discomfort to the patient, and are safer than removable implants. In removable applications, afterloading technics greatly reduce the radiation hazard (1) or even eliminate it altogether (2), but in permanent implants they are of only limited help. The only solution to the radiation exposure problem in permanent implants appears to be the use of lowenergy gamma emitters. Their value in reducing radiation hazard was first pointed out by Harper and his associates in 1958 (3), who suggested radioisotope solutions, especially cesium 131 and palladium 103, either for injection into applicators or for direct injection into tumors. On the other hand, we believe these low-energy radioisotopes can be made more effective by encapsulating them in the form of seeds similar to the commonly employed radon222 and gold-198 seeds. In our experience with many different technics, only encapsulated discrete sources provide the accuracy in distribution and dose level essential for optimal and reproducible results in cancer therapy. In the past years, we have made many efforts to obtain low-energy seeds from suitable artificial radioisotopes (4). After much experimentation, cesium-131 and iodide-125 seeds are now being produced on a limited scale by the Hazleton Nuclear Science Corporation and have been made available for preliminary studies at the Memorial Sloan-Kettering Cancer Center since July 1964. Cesium 131 has a half-life of 9.7 days and emits only characteristic x-rays of 29.4 kev with no beta radiation. It can be produced with fair yield by neutron irradiation of barium, followed by subsequent chemical separation from the 11.6-day barium-131 parent. The presently available cesium-131 seeds consist of a central core to which carrierfree cesium 131 is bound by ion exchange. This core is enclosed in a thin, cold-welded aluminum jacket and an outer heat-sealed Teflon capsule forming a double encapsulated sealed source of 0.8 mm diameter and 4.5 mm total length. A gold filament is being incorporated in the central core to make the seed more visible in radiographs. These cesium-131 seeds withstand multiple autoclaving without leakage, do not cause tissue irritation, can be implanted through the same 17-gauge needles as radon-222 and gold-198 seeds, and can be mass-produced. Spectral analyses of seeds made from the initial processing of irradiated barium showed, besides the 29.4 kev x-ray line of cesium 131, less than 0.02 per cent of the 6.5-day cesium 132, while seeds fabricated from a “second milking” of the barium 131-cesium 131 “cow” disclosed less than 0.0001 per cent of this or other radiocontaminants. Figure 1 illustrates comparative absorption curves in water for our cesium-131 seeds as well as for commercially available radon-222 and gold-198 seeds.