In stereotactic interstitial radiotherapy, small radioactive sources are placed within the brain tumor to deliver locally high radiation doses. The choice of the radioisotope depends upon the dose distribution around the isotope, energy of the emitted radiation, relative biological effectiveness, and finally, the cost and availability of the isotope. We have analyzed 198gold,125iodine and 192iridium in terms of these four factors. Our results have shown that 125I is superior to the other two isotopes due to its soft X-rays and dosimetric as well as radiobiological properties. Unfortunately, it is the most expensive of these radioisotopes, and can be difficult to obtain in specific activities.