During a stereotactic neurosurgical operation, up to 3 thin teflon catheters (external diameter 1.6 mm) are implanted to a high degree of accuracy in the brain tumor in target areas that have been precisely calculated in advance. These catheters contain a certain number of radioactive radiation sources (iodine-125 seeds) that has also been calculated in advance. The radioactive substance iodine-125 is located in a small titanium capsule (dimensions: 4.5 mm x 0.8 mm). An x-ray of very low intensity is emitted from this titanium capsule. The penetration depth of the radiation in the tumor or brain tissue can be regulated very precisely via the strength (=activity) of the seeds.

After fixing the catheters in place under the skin, the tumor is steadily destroyed from the inside out over a period of weeks or months. Due to the extreme concentration of the radiation in the inside of the tumor, significantly higher radiation doses (up to 350 Gy) can be applied to the tissue of the tumor than is possible with conventional external radiation techniques.

Apart from a few exceptions, in all cases, prior to the implantation of the seed, the type of tumor and the level of malignancy will need to be determined by means of a stereotactic biopsy. Once the sources of radiation have been implanted there is no need to comply with any complicated radioprotection rules.