Low Dosage Rates Research Articles

A 250-curie caesium 137 unit designed for low dosage rates at short source to skin distances.

A caesium 137 unit containing 60 c in a source of diameter 1·5 cm is described. It gives rectangular and circular fields up to 10 × 10 cm. The treatment distance is 15 cm. The dose-rate on the skin varies from 35 to 38 r/minute, according to the field size. The isodose distributions obtained with the unit are identical with those from a cobalt 60 unit at the same source to skin distance. Because of the longer half-life of caesium 137, the unit appears to offer some advantages over cobalt 60. It can be made much smaller than a corresponding unit using cobalt 60.

The Effect of Low Dosage X-Ray Irradiation on Reproduction in Chickens

IT IS well known that the functional ability of the ovary is interfered with by high dosage x-ray irradiation (Bloom, 1948). It is also known that the extent of damage is greatly influenced by dosage rate and over-all time of exposure. What is not known is whether the sub-threshold x-ray dosage levels delivered at a low dosage rate have any long range effect on the reproduction potential of the ovary. Since the White Leghorn chicken has a long and intensively active reproductive period it is an excellent organism on which to study these factors.It was the purpose of the investigation reported here to study the effects of low dosage x-ray irradiation on total egg production and the fertility and hatchability of the eggs produced. The effect of age at radiation on over-all response was also studied by irradiating chickens before and after sexual maturity. Finally, line differences in response …

Protracted interstitial irradiation of tumours using 182Ta.

Many radiotherapists believe that protracted or fractionated courses of radiotherapy are more effective in the treatment of malignant growths than courses of shorter duration. It has been suggested that the average skin tolerance dose varies roughly as the cube-root of the overall time (T0·33), while the median lethal dose for epidermoid cancer depends on a factor between T0·22 (Strandqvist, 1944) and T0·25 (Cohen and Kerrich, 1951). For practical reasons a course of radiotherapy is rarely protracted longer than six to eight weeks and if we assume that Strandqvist's formula is at least approximately applicable, the therapeutic ratio (here defined as the ratio of normal tissue tolerance dose to the median tumour lethal dose) increases by no more than T0·33−0·22, that is the ninth-root of the overall time. A significant improvement in this ratio could be obtained only with very much longer treatment times. By using permanent implanes of relatively weak sources of moderately long-lived, γ-ray emitting, chemically inert radionucleides, it is possible to deliver extremely protracted continuous irradiation at unusually low dosage rates. The equivalent overall treatment time and the corresponding optimal dosage for an exponentially decaying source is readily calculated (Cohen, 1950).

Further Studies on the Effects of Irradiation on Proliferation and Metabolic Processes of Normal and Malignant Tissues

THE time-intensity factor related to the effectiveness of irradiation upon living processes remains to-day an unsettled problem among the investigators in this specialized field. The question as to whether high dosage rates for a short time or low dosage rates for a long period of exposure produce more marked effects of radiation has not yet been definitely answered. The present series of experiments was undertaken in an attempt to throw some light upon this question. Before presenting the results obtained from these experiments, reference will be made to our previous investigation (1). The determination of the lethal and sublethal doses produced by different types of radiation for various tissues grown in vitro was chosen as a basis for that investigation. The following technic and criteria were used: Freshly excised tissue fragments embedded in a nutritive medium on a No. 1 coverslip and covered with a mica sheet were inverted over a hollow ground slide, sealed with paraffin wax, and irradiated. The irr...