Proportion Of Hypoxic Cells Research Articles

The influence of intracellular recovery and hypoxic cells on the radiation response of mammary tumours and skin in C3H mice.

Spontaneous and transplanted tumours in the C3H mouse have been irradiated in oxygen at one atmosphere absolute using either single doses or doses split into two equal fractions with the time interval between them being varied from one hour to three days. The response of the tumours was compared with that of the skin over and around the tumour as representing normal tissue. Intracellular recovery for both tumour types, shown by a positive (D2–D1) 6 hours was seen at all doses studied, and evidence was given for recovery in the hypoxic proportion of the tumour. The skin showed a gradual decrease in (D2–D1) 6 hours as the dose increased, until after an initial dose of 2,750 rads no intracellular recovery was seen with a negative (D2–D1) at all intervals up to three days. This and the sensitisation in the tumours seen at longer time intervals (i.e. negative (D2–D1)) are interpreted as reoxygenation phenomena, or change in proportion of hypoxic cells. The rate and amount of reoxygenation were observed to be greater in spontaneous than in transplanted tumours. The therapeutic advantages of short time intervals are discussed.

The relative effects of fast neutrons and x rays on tumour and normal tissue in the rat. II. Fractionation: recovery and reoxygenation.

1. In rats an experimental sarcoma, known to contain hypoxic cells, and the normal tissue of the foot have been irradiated with fractionated doses of X rays and of fast neutrons, each dose separated by 24 hours. 2. For equal damage to normal tissue the therapeutic effect of X rays is improved by fractionation, but in this experimental system neutrons are always still more effective on the tumour. 3. This result is explained by the fact that the population of tumour cells surviving a dose of radiation is largely hypoxic, but that the proportion of hypoxic cells is reduced in time. However, at 24 hours after irradiation this proportion is still greater than in the untreated tumour. 4. Late damage and deformity of the normal tissue, relative relative to early damage, is the same for single doses, two or five fractions of X rays or of neutrons. However, for all treatments, once above a certain dose level, a small increase led to a small increase in early reaction, followed by a very large increase in late reaction and deformity. 5. The rate of increase of neutron RBE with increasing numbers of fractions is similar to that found in pig skin and in human skin. 6. By comparison with the single dose, “recovery” after neutron irradiation is less than after X rays by 0·4 for the early skin reaction and by 0·6 for the late reaction and for deformity.

Effect of a Single Dose of Radiation on Proportion of Hypoxic Cells in a C3H Mouse Mammary Carcinoma

A current interest in radiobiology is the investigation of shifts in the distribution of oxygen tension in solid tumors during the course of fractionated radiation therapy. If upward shifts in oxygen tension occur, the radiobiological significance of a small focus of hypoxic cells in the untreated tumor could be drastically reduced. Examinations for such shifts in pO2 distributions within tumor tissue in man have been based on the polarographic technic with use of very small electrodes, i.e., ≈50–100 µ in diameter. Results have been reported from three laboratories, however, and they have not been consistent (1–3). In contrast to these equivocal results from Polarographic studies at the clinical level, experimental data from three laboratories strongly indicate that during fractionated irradiation there is an improved oxygenation of tumor tissue (4–8). First, studies of a mouse mammary carcinoma showed that an appreciably larger fraction of cells responded as though they were aerobic when radiation was administered in ten equal doses than when a single dose was given (4). Similar findings were obtained in studies of a squamous-cell carcinoma and a fibrosarcoma (5). Van Putten and Kallman (6) investigated a C3H mouse fibrosarcoma and found that during the course of fractionated irradiation (200 R daily × 5), the hypoxic cell population decreased much more rapidly than expected, indicating a shift of pO2 tensions during the five-day treatment. Similarly, Thomlinson (7) analyzed the changes in the growth curves of a benzpyrene-induced fibrosarcoma (transplanted into inbred rats). He employed one and two doses of radiation administered under conditions of hypoxia, air, or respiration of oxygen at high pressure. Results were interpreted as showing that following a dose of radiation there is a very rapid increase in tumor tissue pO2. In addition, microradiographic studies on rat tumors before and after irradiation were interpreted by Rubin (9) as indicating a decrease in intercapillary distances following irradiation. The present report describes an experiment which was planned to estimate the extent of transfer of cells out of the hypoxic compartment during the first five days following a single radiation dose to a mouse mammary carcinoma. Materials and Methods Animal Tumor System: These experiments were performed on third generation transplants from a single C3H/He mouse mammary carcinoma. A previous report (10) presented details of: origin and maintenance of this tumor; technic of preparation of suspension of tumor cells; counting of viable cells. For these experiments an inoculum of 5–10 µl containing 105 viable tumor cells was injected into the muscle mass of the distal right thigh. Experimental Animals: As recipients of the tumor transplantation we used (C3H/He × C57)F1 hybrid mice, designated hereafter as C3BF1, obtained from the Texas Inbred Mouse Company, Houston, Texas.