[The dosage-performance effects on Ca-Ski and HPK cells in relation to the dose and fractionation]

The steep decrease of dose and dose-rate in brachytherapy implies very different radiobiological considerations of the biological effectivity.

Therefore, in imitation of the clinical procedure, we compared the LDR-, MDR- and HDR-brachytherapy. We carried out experiments on epidermoid cervix carcinoma cells (Ca-Ski cells) and human primary keratinocytes (HPK cells) obtained after transfection with human papillomavirus type 16 DNA varying the dose-rate (28 cGy/h to 8000 cGy/h), the dose (1 Gy to 100 Gy) and fractionating (protracted, 3, 6 and 12 fractions).

1. At dose-rates of 75 cGy/h (Ca-Ski cells) and 110 cGy/h (HPK cells) respectively we found that the cells "fall asleep" at doses up to 100 Gy; the rate of cell mortality is insignificantly higher than the proliferation rate. 2. A first-time proof of an accumulation of repopulation effects (recovery from the sublethal radiation damage, progression of the cells during the partial cycle/proliferation and the acts of redistribution), if the radiation exposure reaches the median time of the cell cycle (HPK cells, dose-rate: 110 cGy/h, doses: 1 Gy to 100 Gy). 3. Each increase in the dose-rate requires higher fractionating. At dose-rates higher than 300 cGy/h (range of a percutaneous radiotherapy), we found that the survival rates of the cells could only be increased insignificantly in spite of a fractionated therapy (3, 6 or 12 fractions; doses: 1 Gy to 100 Gy); the repopulation effects almost vanished.

Changing a LDR- into an HDR-brachytherapy the equivalent factors close to the source have to be selected low and with increasing distances from the source high respectively higher-the major problem for a mathematical formula. The reduction of the dose in HDR-radiation therapy is a compromise in order to limit side effects caused by a radiation. The trade-off is a small therapeutic range and reduced therapeutic effectivity at the tumor. The percutaneous dose at the pelvis wall has to be reduced if at the same time an HDR-brachytherapy will be carried out-to avoid side effects.