Investigations concerning the application of PDR-afterloading-therapy

Introduction: Despite the gain in experiences concerning the PDR-afterloading technology there are still some important questions about its application: Questions: 1. Are the radiation reactions of PDR with Ir-192 and LDR with Co-60 nearly the same? 2. In which way the PDR-regimen has to be modified in case of tumor volume < 20·20·10 mm3, e.g. brachytherapy in the head and neck region? 3. Is it possible to interrupt the PDR-therapy during the night? Material and methods: We compared the radiation reactions of LDR-technique and two PDR-regimens by means of the guinea pig skin model (20 female animals) The following regimens are used: 1. continuos LDR-regimen 30 Gy in 60 hours 2. PDR-regimen 0,5 Gy hourly pulse length 10 minutes, 30 Gy in 60 hours 3. PDR-regimen with 9 hours night break 30 Gy/60 hours The isodose distribution patterns of the LDR-source (Co-60 source with an activity of 2 GBq) and the PDR-source (Ir-192 source with an activity of nearly 15 GBq) are the same. We implanted an interstitial applicator on the right hand side of the animal back with a Co-60 source and on the left hand side an interstitial applicator for a PDR-irradiation. The radiation reactions were controlled by the help of an evaluation table in which the criteria of radiation reaction - exsudation, hair growth and atrophy - were classified according to the degree of seriousness. The observation time is maximal 21 months. The degrees of seriousness are presented graphically in diagrams which we analysed by means of the cross-correlation-function. Results and conclusions: 1. Generally, it is possible to compare the radiation reactions of PDR irradiation and the classic continuos LDR brachytherapy with Co-60-sources. Calculations with the aid of the Monte-Carlo-method resulted in the fact that the gamma dose is the same in a distance of 2 mm from the source. 2. The activity of the source corresponds to the volume of tumor: the smaller the volume of the tumor the lower the dose rate (activity) of the source. 3. The results show that it is possible to use a PDR-regimen with a night break of 9 hours