THE INFLUENCE OF MINERALIZING RADIONECROSIS ON THE DOSE DISTRIBUTION IN INTERSTITIAL RADIATION-THERAPY OF BRAIN-TUMORS

The influence of radionecroses arising during interstitial radiation of brain tumours with I-125, Ir-192 or Au-198 on dose distribution was investigated using Monte Carlo methods. The necroses have a higher density than normal tissue due to radiation-induced changes in tissue composition as well as mineral deposits. They can have a diameter of up to 1 cm around the single seeds. The higher density and changed chemical composition compared to homogeneous normal tissue leads to increased absorption of radiation around the necroses which results in a lower dose rate in the surrounding tissue. It is shown that the formation of necroses during treatment with higher energy radiation such as Ir-192 (340 keV) or Au-198 (400 keV) may be neglected during therapy planning as the dose rate is affected by less than 2%. If low energy radiation, e.g. I-125 (28 keV) is used, the dose rate can be reduced by more than 30%. In this case the influence of the necroses on dose distribution, at least for permanent I-125 implantation, may not be negligible.