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[The initial results on the radiobiological comparability of continuous LDR irradiation and PDR irradiation using a guinea pig skin animal model].

AIMS
The classic continuous low dose rate (LDR) brachytherapy was very important in such cases with a higher risk of severe especially late radiation reactions. From clinical experiences and radiobiological considerations it is known, that the therapeutic ratio of LDR is higher than HDR. Another way to combine the therapeutic ratio of LDR and the possibility of optimisation is the use of pulse dose rate (PDR) technique. The PDR-technique is a method, which can be compared with continuous LDR-therapy. PDR should have biological effects equivalent to conventional LDR.


MATERIAL AND METHODS
We have tried to compare the classic continuous LDR-technique with 2 PDR-regimes by means of the guinea pig skin model. In this test series we involved 20 female animals with an initial weight of 400 to 500 g. We compared radiation reactions of following regimes: 1. Continuous LDR regimen with a cobalt-60 source with an activity of 5.5 mCi 30 Gy in 60 hours. 2. PDR regimen 0.5 Gy hourly, pulse length minimal 10 minutes, 30 Gy in 60 hours with an Ir-192 source with an activity of nearly 40 mCi. 3. PDR-regimen 0.8 Gy hourly with 9 hours night break (10.00 p. m. to 7.00 a. m.). The radiation reaction was controlled by the help of an evaluation table in which the criteria of radiation reaction were classified according to the degree of seriousness. The observation time is now minimal 14 and maximal 24 months.


RESULTS
The findings shows a significant coincidence of early and late radiation reactions of the skin fields irradiated with the continuous LDR-technique and fields irradiated with the PDR-technique. There was not a difference of the radiation reactions between PDR-irradiation with and without night break.


CONCLUSIONS
Generally it is possible to compare the radiation reactions of PDR-irradiation and the classic continuous LDR-brachytherapy. It is also possible to use a PDR-regimen with a night break of 9 hours. But results must be calculated for each tissue of interest, in our test consequently for guinea pig skin.