A study of the relative biological effectiveness for 35-MEV electrons with mammalian cells (100 percent-10 percent depth-dose levels)

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.

RBE of 20 kV and 70 kV X-rays determined for survival of V 79 cells

The survival curves of V 79 cells were compared for various types of sparsely ionizing radiation: 20 kV, 70 kV and 42 MV X-rays, 60Co gamma-rays and 40 MeV electrons. A carefully calibrated thin window chamber was used for accurate dosimetry. The survival curves for cells exposed to 20 kV X-rays and 70 kV X-rays differed from that for 60Co gamma-rays only by a dose-scaling factor t(RBE). RBE with respect to 60Co gamma-rays could then be considered as dose-independent: 1.19 +/- 0.06 for 70 kV X-rays and 1.40 +/- 0.07 for 20 kV X-rays. RBE of 42 MV X-rays and 40 MeV electrons was not significantly different from 1.