[Effects of fractionation and dose rate in PDR brachytherapy of B14 cells]. / Effekte von Fraktionierung und Dosisleistung bei der PDR-Brachytherapie von B14-Zellen.

PURPOSE: Present radiobiological studies for different cell lines in vitro demonstrate the equivalence and efficacy of continuous low-dose-rate brachytherapy (LDR-BT) and pulsed dose rate brachytherapy (PDR-BT) when using small and frequent dose pulses. The aim of this study was to examine monolayer fibroblast cultures in vitro to examine the biological effects of different pulse doses and dose rates under clinically conditions. MATERIAL AND METHODS: B14 cells, Hy B14 FAF 28, peritoneal fibroblasts, were cultured in multi-well plates and exposed to a PDR radiation source at a distance of 9 mm. The following PDR-schemes were compared: dose per pulse: 1 Gy, 2.5 Gy and 5 Gy to a total dose of 5 Gy/5 h (overall time), 10 Gy/10 h, 20 Gy/20 h and 30 Gy/30 h. The pulse duration for the examination of dose rate effects was 20 min, 30 min or 52 min corresponding by dye pulse dose rate of 300 cGy/h, 200 cGy/h or 115 cGy/h. Treatment endpoints were cell measured by dye exclusion test and clonogenic cell survival. RESULTS: Cell survival decreased for pulse doses of 5 Gy compared to 2.5 Gy or 1 Gy per pulse (mean dose rate 200 to 300 cGy/h). No differences were observed with dose rates during irradiation of 300 cGy/h, 200 cGy/h or 115 cGy/h (20 Gy/1 Gy). CONCLUSION: Radiobiological effects of PDR-RT are dependent on the dose per pulse, with differences in biological effects only with a dose per pulse of more than 2.5 Gy, considering the described in-vitro conditions. More examinations with a more pronounced difference in dose rate will be continued for evaluation of dose rate effects.

[The dosage-performance effects on Ca-Ski and HPK cells in relation to the dose and fractionation]. / Dosisleistungseffekte an Ca-Ski- und HPK-Zellen in Abhängigkeit von Dosis und Fraktionierung.

BACKGROUND: The steep decrease of dose and dose-rate in brachytherapy implies very different radiobiological considerations of the biological effectivity. MATERIAL AND METHODS: 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). RESULTS: 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. CONCLUSIONS: 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.

Fiber-optic transmission of stretched pulses from a Q-switched ruby laser.

Fiber-optic transmission of Q-switched ruby laser pulses is limited by fiber damage owing to the high laser-beam intensities. Pulse stretching with a semiconductor-based control circuit for the Pockels cell of the ruby laser to reduce the peak intensities is described. Pulses with durations from 200 ns to 1 µs and a coherence length of ~3 m were generated. These pulses were coupled into multimode optical fibers to investigate the transmission characteristics and the limits of transmittable pulse energies. Stretched pulses can be transmitted in quartz fibers with a 600-µm core diameter to pulse energies of 300 mJ, which is an increase by a factor of 4 compared with standard Q-switched pulses. It is expected that beam guiding of ruby laser pulses by fiber optics will significantly facilitate the use of holographic interferometry in technical applications such as vibration analysis.