One reason why cobalt-60 is preferable to cesium-137 as radioisotopes in the Leksell Gamma Knife, in spite of its lower half-life (5.26 years versus 30.0 years), is its much higher specific activity. Therefore, smaller physical source size can be used and still enables the Leksell Gamma Knife to produce a high radiation dose output with small geometrical penumbra. The smaller geometrical penumbra is essential to preserve more normal brain structures.

     In this study, we employed the cesium-137 as radioisotope in the Leksell Gamma Knife sources and investigated its dosimetric properties in the Gamma Knife radiosurgery.

     The Monte Carlo system employed was the PRESTA (Parameter Reduced Electron-Step Transport Algorithm) version of the EGS4 (Electron Gamma Shower) computer code. The cesium-137 model CDCS.J type tubes (Amersham Healthcare Medi-Physics, Inc., Arlington Heights, IL.) have external diameters of 2.65 mm with physical lengths of 20 mm and active lengths of about 14 mm.

     Single beam dose profiles of 4mm, 8mm, 14mm and 18mm were calculated for cesium-137 and cobalt-60. The PDD curves for the 18mm collimator were also generated for comparisons.

     The single beam profiles for cesium-137 showed slightly smaller penumbra regions than cobalt-60. The results were particular significant for the smaller collimator sizes. The reason is that cesium-137 has lower photon energy than cobalt-60 and hence its short electron range. The PDD curves of cobalt-60 generally exhibit somewhat less absorption of radiation than cesium-137. It is because the relative low photon energy of cesium-137 behaves less penetration power than cobalt-60.

     This study did not evaluate radiobiologic effects.

     The advantages in using cesium-137 as radioisotopes in the Leksell Gamma Knife sources are its long half-life and its smaller penumbra regions.

     However, the Leksell Gamma Knife models are unable to use cesium-137 because the source volume is huge which worsen the geometric penumbra.