F.A. Stewart

The therapeutic advantage of combined heat and X rays on a mouse fibrosarcoma

The response of an experimental fibrosarcoma to combined heat and X rays has been assessed using delay in tumour regrowth. No thermal sensitization was observed for one hour of heating at 41.5 degrees C. A thermal enhancement ratio of 1.4 to 1.5 was seen for one hour of heating at 42.5 or 43.5 degrees C immediately after irradiation. The importance of the sequence and time interval between the two modalities was tested using a constant heat treatment of 42.5 degrees C for one hour. Heating was most effective when given after irradiation, with little change in TER for an interval of a half to six hours. When heat preceded the irradiation the pattern was more complex, with no sensitization at half, one, and six hours, but with a significant effect at two and three hours. The degree of sensitization often depended upon the X-ray dose, being lower below X-ray doses of about 20 Gy. The therapeutic advantage of the combined treatment was assessed by comparison with previously published results for skin reactions. No therapeutic benefit was found for treatments when heat and X-rays were separated by one hour or less. A therapeutic gain factor of about 1.3 was observed for heat given two to six hours after irradiation of this fibrosarcoma.

Proliferation kinetics of the mouse bladder after irradiation.

The proliferative response of the mouse bladder was investigated, using continuous labelling with tritiated thymidine, at various times after a single dose of radiation. Bladder epithelial and vascular endothelial cells were studied. The cell turnover rate in unirradiated epithelium and endothelium was found to be extremely slow (in excess of 1 year).

Changes in the proliferation rate of mouse epidermis after irradiation. Continuous labelling studies.

The proliferative response of mouse skin to damage caused by X‐irradiation has been tested by giving repeated injections of tritiated thymidine and scoring the percentage of labelled cells in high resolution autoradiographs. Four, nine and fourteen daily fractions of 300 rads of X‐rays were used and labelling commenced 4 days after the last fraction. The epidermis of the upper surface and the sole of the foot were scored separately and were compared with the skin of unirradiated feet.

Sensitization of mouse skin to x irradiation by moderate heating

The combined effects of heat and x irradiation have been studied on mouse skin using skin reactions on the feet. The skin was heated for one hour by immersion in a water bath at temperatures ranging from 25.5-43.5 degrees C. When the x irradiation was in the middle of this heating period a thermal sensitization to the x-ray damage was observed. The effect increased with temperature above 39.5 degrees C and doubled the effectiveness of the x rays at 43.5 degrees C. When the heating and irradiation were not concurrent the timing and sequence of treatments were important. Heat before irradiation was more effective than heat after irradiation, particularly if the interval was three hours or greater.

Radioprotection of mouse skin by WR-2721: The critical influence of oxygen tension

The epidermal clone assay has been used to study the radioprotective effect of WR-2721 on mouse skin under different conditions of oxygenation and under anoxia. The skin has shown a progressive decrease in sensitivity as the inspired gas was changed from 100% oxygen towards 0% oxygen. Compared with mice breathing 100% oxygen, those breathing air are partially protected. The inspired oxygen concentration to given half the full oxygen effect is 10--12%. The radioprotection observed with 400 mg/kg WR-2721 is markedly dependent on the ambient oxygen concentration. The protection factor is 1.1 or less in mice breathing 5%, 1% of 0% oxygen. Protection is maximal (1.95) in air and in 50% oxygen and diminishes to 1.6 at higher oxygen tensions.

Radioprotection of two mouse tumors by WR-2721 in single and fractionated treatments

The radioprotective effect of WR-2721 has been studied in two murine tumors using single doses or five daily fractions. Single dose irradiations of the SA FA resulted in a highly variable radio-protective response. In one experiment large protection factors (1.2-2.5) were obtained, with the greatest protection at low X ray doses. In later experiments with the same tumor, there was little or no radioprotection. The Ca MT was significantly protected against single dose irradiation with both 250 mg/kg and 400 mg/kg of the drug. In a five fraction schedule the extent of radioprotection for CA MT was greater than with single X ray doses for the same drug dose per fraction. Tumor protection factors from the present work and from the literature are compared with published protection factors for normal tissues. Significant tumor radioprotection is seen in most studies. The data indicate more variability in the extent of tumor protection for a given drug dose than is seen in normal tissues. Tumor protection is often greatest at low X ray doses which may be a result of preferential protection of the better-oxygenated tumor cells.

Fractionation studies with WR-2721: Normal tissues and tumour

We have studied the ability of WR-2721 to protect skin, kidney and an anaplastic murine tumour against single or fractionated X-ray treatments. Skin reactions, four different kidney assays, regrowth delay and local control of tumours have been used to construct dose-response curves from which the degree of radioprotection can be quantified as a protection factor. Low doses of WR-2721 (0.2-0.3 mg X g-1) were used before each of 1, 5 or 10 fractions. The degree of protection was similar in all three systems and it did not change significantly with fractionation.

Multifraction irradiation of mouse bladders

The response of mouse bladders to multifraction irradiation was assessed from increases in urination frequency or the reduction in bladder capacity after irradiation. A range of electron doses were given as 1, 2, 5, 10 or 20 equal fractions in overall treatment times of 1-2 weeks. Dose-related increases in urination frequency were measured from 10 to 14 months after irradiation and a dose-related reduction in bladder capacity (at inflation pressures of 20 mm Hg) was apparent at the time of sacrifice. The extent of repair of sublethal and potentially lethal damage was estimated from a comparison of the isoeffective doses in fractionated regimes and single dose treatments. After small doses per fraction (2.5-6 Gy), the extent of repair in bladder was very similar to that in mouse skin. After larger doses per fraction (greater than 8 Gy) slightly more repair was seen in bladder than skin. Linear-quadratic analysis of the data suggests quite a high value for the ratio alpha/beta, in the region of 5 to 10 Gy. This is higher than the alpha/beta ratios which have been reported for most other slowly dividing normal tissues.

Evidence for repair capacity in mouse tumors relative to skin

Abstract The repair capacity of six different types of mouse tumor has been tested by comparing regrowth delay curves for single doses and two or five fractions. The problem of reoxygenation has been avoided by making the tumors uniformly resistant or sensitive for each irradiation. The repair increments measured were mostly lower in the tumors than in skin. The data were analyzed in terms of the fractional recovered dose (F R ). F R values for these tumors at low doses averaged 20–50% compared with 65–90% for skin. It is postulated that this may result from a difference in the initial slope of the survival curve of tumor cells relative to normal cells, rather than from a difference in the value of D Q .

Repair during fractionated irradiation of the mouse bladder

The capacity for repair of sublethal damage during fractionated irradiation of the mouse bladder has been measured. Graded doses of electrons were given as 1, 2, or 5 equal daily fractions. Two functional end points were used to assess bladder damage: (a) increased urination frequency and b) decreased bladder capacity. Repair of sublethal injury within 24 hours was found to be similar using both assays for bladder damage and was greater than the repair observed in mouse skin for a given dose per fraction. The possibility of a slower repair process occurring in bladder was investigated by giving two fractions in increasing overall times (from 24 hours to one month). No increased repair was observed with the longer time intervals; hence there was no evidence for slow repair in the bladder.