Continuous Low Dose Rate Irradiation Research Articles

The response of pig skin to single and fractionated high dose-rate and continuous low dose-rate 137Cs-irradiation—II. Theoretical considerations of the results

The result of a comparison of single and fractionated irradiation at high dose rate with continuous irradiation at low dose rate on pig skin is discussed from a radiobiological point of view. A short review is given of other fractionation and dose-rate investigations. Some dose-rate experiments have resulted in an increased therapeutic gain for continuous irradiation at low dose rate, compared to single acute irradiation. However, there is a lack of data comparing fractionated high dose-rate irradiation with continuous low dose-rate irradiation on normal tissues as well as malignant tumors.

The response of pig skin to single and fractionated high dose-rate and continuous low dose-rate 137Cs-irradiation—I: Experimental design and results

A comparison of the radiation effect of single and fractionated irradiation at high dose rate (1.48 Gy/min) with continuous irradiation at low dose rate (1.2 Gy/hr) was performed on pig skin. Two identical plane 137Cs-sources with similar geometrical arrangement were used, giving about the same field flatness and depth-dose distribution. Dose-response curves were established for the 3 irradiation modalities. The dose-rate factor between single doses given with high and low dose-rate irradiation was found to be 2.35 for the medium-term skin reaction. These experimental data also make it possible to establish an equivalent biological scale of radiation effect on normal tissue by the cumulative radiation effect (CRE) system over a wide range of irradiation techniques, including fractionated high dose-rate and continuous low dose-rate irradiation regimens. The value of normalizing factor in the CRE-formula for continuous low dose-rate regimens was found to be significantly lower than proposed.

The destiny of brachytherapy in oncology

At the invitation of the American Radium Society, Pierquin reviews his very large experience with continuous low dose rate irradiation of cancer. The target volume is treated with 6,000-7,000 rad in 3-10 days at a dose rate of 30-100 rad/hr. Preference for continuous low dose rate irradiation is based on the earlier work of Jean Pierquin, Georges Richard, Claudius Regaud, and Ralston Paterson with radium sources. Afterloading techniques with iridium-192 have been substituted allowing safer and more precise placement. An accumulated experience with 5,000 patients over the past 15 years permits conclusions that the techniques are safe, easy, inexpensive, and effective for tumors accessible to direct instrumentation. Dosimetry is based on the Paris system. Standardizing upon a dose of 7,000 rad, Pierquin concludes there is an isoeffect of continuous radiation at low dose rates of 25-100 rad/hr. The differential effect on tumor versus healthy tissue is impressive even when large volumes are irradiated, as in neck adenopathy. Stemming from this experience, three trials have been carried out of low dose rate teletherapy with cobalt-60 sources in oral cavity and oropharynx tumors, delivering 7,000 rad at rates of 90-100 rad/hr. Pierquin hopes for the same very high local control rates (in the order of 95%)achieved by iridium-192 endoradiotherapy of accessible T1, T2, T3, and T4 cancers.

Theoretical considerations of dose rate factors influencing radiation strategy.

A model is proposed under which the effects of regimens of both fractionated high dose rate and continuous low dose rate irradiation can be simulated. Primary features of the model are cycle-related fluctuations in radiation response and the existence of a “single hit” killing component. The effects on cell killing of the dose rate or fractionation regimen chosen and of the proliferation kinetics of the tissue are examined. It is not possible to convert from a continuous regimen to a supposedly equally effective regimen of daily fractions unless the tissue kinetics are considered.

Spermatogonial Cell Population Kinetics under Continuous Low Dose Rate Irradiation

Spermatogonial Cell Population Kinetics under Continuous Low Dose Rate Irradiation

Susceptibility to infection with Pasteurella tularensis and the immune response of mice exposed to continous low dose rate gamma radiation.

Abstract : Mice were exposed to continuous low dose rate (1.4 R/hr) gamma radiation. Immediately after accumulating 1000 R, 2000 R, or 3000 R, they received a subcutaneous injection of the live vaccine strain (LVS) of Pasteurella tularensis. Although the avirulent strain caused no effect in normal mice, a fulminating infection occurred in irradiated mice. As the total radiation dose increased the LD50 of the organism decreased. Animals surviving irradiation and immunization were subjected to an aerosol challenge of either the avirulent LVS or the virulent SCHU S-5 strain of P. tularensis. The LVS strain is virulent for mice when administered as an aerosol, but not when injected subcutaneously. Although an aerosol of avirulent organisms was lethal for normal mice, immune mice, both non-irradiated and irradiated, survived an aerosol infection with the avirulent organism. Irradiated, immunized mice challenged with an aerosol of the virulent SCHU S-5 strain showed a decrease in immunity, especially at the higher levels of radiation. Bacterial counts of the lungs, liver, spleen, and lymph nodes of non-immunized mice exposed to a lethal aerosol dose of the avirulent P. tularensis (LVS) showed increasing numbers of bacteria after the 2nd post-infection day. This increase continued until the death of the animal on about the 10th day. Immune animals, however, were able to clear the lungs 5 days after infection with no involvement of the other tissues. Although continuous low dose rate gamma radiation does not produce the gross radiation pathology that one sees following acute radiation, exposure to a live avirulent Pasteurella tularensis vaccine produces an infection and an impaired immunity.