Relative biological effectiveness of high-energy photons (up to 50 MV) and electrons (50 MeV).

In vitro studies of the relative biological effectiveness (RBE) of 50-MV X rays have shown an RBE of 1.1 relative to 4-MV X rays. This will be important in clinical radiotherapy. The aim of this study was to verify these results and to investigate whether photonuclear processes might cause the difference in RBE. To do so, 50- and 20-MV X rays and 50-MeV electrons were investigated with respect to RBE. Chinese hamster V79 cells were irradiated in a specially designed system which allows for a high reproducibility of geometry and dosimetry. Fractionation experiments were also carried out to establish the RBE at the clinically relevant dose level, 2 Gy. Fricke dosimetry was used, and the results were confirmed with ionization chamber measurements. The RBE for 50-MV X rays was estimated to be 1.14 at a surviving fraction of 0.1 and 1.12 at a surviving fraction of 0.01. The RBEs for the other qualities were equal to one. The RBE calculated for the 2 Gy/fraction experiments was 1.17.

Biological response in vitro to pulsed high dose rate electrons from a clinical accelerator.

Several studies of biological response to ionizing radiation of high absorbed dose rates have been performed, often with conflicting results. The aim of this study was to establish whether a difference between irradiation at high dose rates and at more conventional dose rates could be verified. Pulsed 50 MeV electrons from a clinical accelerator were used both for the high dose rate experiments (mean dose rate: 3.8 x 10(2) Gy/s) and the reference experiments (mean dose rate: 9.6 x 10(-2) Gy/s). In this study V-79 cells were irradiated in vitro. The experiments were carried out under both oxic and anoxic conditions. No significant difference in relative biological effectiveness (RBE) or oxygen enhancement ratio (OER) was observed at the different dose rates investigated.

Enhancement of radiation sensitivity in iododeoxyuridine labelled cells exposed to low energy x-rays.

In order to investigate if low-energy x-rays induce Auger cascades by photoelectric absorption in iodine present in DNA, CHO cells were labelled with iododeoxyuridine (IUdR) for 72 hours. Following labelling, the cells were either irradiated with low-energy x-rays (75 kV, 4 mm Al) or 137Cs-gamma-rays. The radiation response was measured using clonogenic survival, and the survival parameters were analyzed according to the linear quadratic model. The dose modifying factors were determined as the ratios of the alpha-coefficients. The IUdR labelled cells were found to be about 3.2 times as sensitive as the control cells when irradiated with low-energy x-rays. For 137Cs-gamma the ratio was about 1.5. The standard deviations were estimated by Gauss' approximation to be about 0.5 for both irradiation conditions.

Relative biological effectiveness and oxygen enhancement ratio of 50 MV X-rays.

Experiments in vitro, using V-79 Chinese hamster fibroblasts, were carried out in order to establish whether or not there was a difference between the relative biological efficiency (RBE) and/or the oxygen enhancement ratio (OER) of 50 and 4 MV bremsstrahlung photons. Dosimetry was performed with both Fricke dosimetry and ionization chamber. Cells were irradiated in all glass-metal system under both oxic and anoxic conditions and subsequently plated for cloning assay. Dose corrections were made for the backscatter from the glass to which the cells were attached during irradiation. A statistically significant difference between RBE of the two energies was found. The RBE was estimated to be 1.1 for 50 MV photons. No difference in OER was found. This RBE difference is of clinical interest as it is generally accepted that it should be possible to determine the dose level with a precision of about +/- 5% in radiation treatment.